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The Contemporary Science Series.
Edited by Havelock Ellis.
VOLCANOES: PAST AND PRESENT.
VOLCANOES: PAST AND PRESENT.
by
EDWARD HULL, M.A., LL.D., F.R.S. Examiner in Geology to the University of London.
With 41 Illustrations and 4 Plates of Rock-Sections.
London: Walter Scott, Limited, 24, Warwick Lane, Paternoster Row. 1892.
BY THE SAME AUTHOR.
The Coal-fields of Great Britain: their History, Structure, and Resources. 4th edit. (1881.) E. Stanford.
The Physical History of the British Isles. With a Dissertation on the Origin of Western Europe and of the Atlantic Ocean. (1882.) E. Stanford.
The Physical Geology and Geography of Ireland. 2nd edit. (1891.) E. Stanford.
Treatise on the Building and Ornamental Stones of Great Britain and Foreign Countries. (1872.) Macmillan and Co.
Memoir on the Physical Geology and Geography of Arabia-Petraea, Palestine, and adjoining Districts. (1886.) Committee of the Palestine Exploration Fund.
Mount Seir, Sinai, and Western Palestine. Being a Narrative of a Scientific Expedition, 1883-84. (1885.) Committee of the Palestine Exploration Fund.
Text-book of Physiography. (1888.) C. W. Deacon and Co.
Sketch of Geological History. (1887.) C. W. Deacon and Co.
PREFACE.
It has not been my object to present in the following pages even an approximately complete description of the volcanic and seismic phenomena of the globe; such an undertaking would involve an amount of labour which few would be bold enough to attempt; nor would it be compatible with the aims of the Contemporary Science Series.
I have rather chosen to illustrate the most recent conclusions regarding the phenomena and origin of volcanic action, by the selection of examples drawn from the districts where these phenomena have been most carefully observed and recorded under the light of modern geological science. I have also endeavoured to show, by illustrations carried back into later geological epochs, how the volcanic phenomena of the present day do not differ in kind, though they may in degree, from those of the past history of our globe. For not only do the modes of eruption of volcanic materials in past geological times resemble those of the present or human epoch, but the materials themselves are so similar in character that it is only in consequence of alterations in structure or composition which the original materials have undergone, since their extrusion, that any important distinctions can be recognised between the volcanic products of recent times and those of earlier periods.
I have, finally, endeavoured to find an answer to two interesting and important questions: (1) Are we now living in an epoch of extraordinary volcanic energy?—a question which such terrible outbursts as we have recently witnessed in Japan, the Malay Archipelago, and even in Italy, naturally suggest; and (2) What is the ultimate cause of volcanic action? On this latter point I am gratified to find that my conclusions are in accordance with those expounded by one who has been appropriately designated "the Nestor of Modern Geology," Professor Prestwich.
Within the last few years the study of the structure and composition of volcanic rocks, by means of the microscope brought to bear on their translucent sections, has added wonderfully to our knowledge of such rocks, and has become a special branch of petrological investigation. Commenced by Sorby, and carried on by Allport, Zirkel, Rosenbusch, Von Lasaulx, Teall, and many more enthusiastic students, it has thrown a flood of light upon our knowledge of the mutual relations of the component minerals of igneous masses, the alteration these minerals have undergone in some cases, and the conditions under which they have been erupted and consolidated. But nothing that has been observed has tended materially to alter conclusions arrived at by other processes of reasoning regarding volcanic phenomena, and for these we have to fall back upon observations conducted in the field on a more or less large scale, and carried on before, during, and after eruptions. Macroscopic and microscopic observations have to go hand in hand in the study of volcanic phenomena.
E. H.
CONTENTS.
PART I.
INTRODUCTION.
PAGE
Chap. I. Historic Notices of Volcanic Action 1-9
" II. Form, Structure, and Composition of Volcanic Mountains 10-19
" III. Lines and Groups of Active Volcanic Vents 20-29
" IV. Mid-ocean Volcanic Islands 30-40
PART II.
EUROPEAN VOLCANOES.
Chap. I. Vesuvius 41-60
" II. Etna 61-68
" III. The Lipari Islands, Stromboli 69-75
" IV. The Santorin Group 76-83
" V. European Extinct or Dormant Volcanoes 84-91
" VI. Extinct Volcanoes of Central France 92-112
" VII. The Volcanic District of the Rhine Valley 13-125
PART III.
DORMANT OR MORIBUND VOLCANOES OF OTHER PARTS OF THE WORLD.
Chap. I. Dormant Volcanoes of Palestine and Arabia 126-135
" II. The Volcanic Regions of North America 136-145
" III. Volcanoes of New Zealand 146-153
PART IV.
TERTIARY VOLCANIC DISTRICTS OF THE BRITISH ISLES.
Chap. I. Antrim 154-159
" II. Succession of Volcanic Eruptions 160-171
" III. Island of Mull and Adjoining Coast 172-176
" IV. Isle of Skye 177-179
" V. The Scuir of Eigg 180-184
" VI. Isle of Staffa 185-186
PART V.
PRE-TERTIARY VOLCANIC ROCKS.
Chap. I. The Deccan Trap-series of India 187-189
" II. Abyssinian Table-lands 190-193
" III. Cape Colony 194-195
" IV. Volcanic Rocks of Past Geological Periods of the British Isles 196-199
PART VI.
SPECIAL VOLCANIC AND SEISMIC PHENOMENA.
Chap. I. The Eruption of Krakatoa in 1883 201-216
" II. Earthquakes 217-224
PART VII.
VOLCANIC AND SEISMIC PROBLEMS.
Chap. I. The Ultimate Cause of Volcanic Action 225-235
" II. Lunar Volcanoes 236-252
" III. Are we Living in an Epoch of Special Volcanic Activity? 253-257
APPENDIX.
A Brief Account of the Principal Varieties of Volcanic Rocks 259-265
Index 268
ILLUSTRATIONS.
Fig. 1. Eruption of Vesuvius, 1872-73 Frontispiece
" 2. Cotopaxi Page 16
" 3. Volcanic Cone of Orizaba " 21
Map of the World, showing Active and Extinct Volcanoes " 23
" 4. Teneriffe, seen from the Ocean " 31
" 5. View of the Summit of Teneriffe " 35
" 6. Probable Aspect of Vesuvius at Beginning of Christian Era " 43
" 7. View of Vesuvius before 1767 " 50
" 8. Map of District bordering Bay of Naples " 52
" 9. View of Vesuvius in 1872 " 53
" 10. Ideal Section through Etna " 63
" 11. Map of the Lipari Islands " 70
" 12. The Island of Vulcano in Eruption " 71
" 13. Ideal Section through Gulf of Santorin " 76
" 14. Bird's-eye View of Gulf of Santorin " 79
" 15. Ground Plan of Rocca Monfina " 80
" 16. Geological Section of Tiber Valley at Rome " 88
" 17. Generalised Section Through the Vale of Clermont " 93
Fig. 18. View of Puy de Dome and Neighbouring Volcanoes Page 95
" 19. Mont Demise, seen from the S.E. " 103
" 20. Sketch Map of Rhenish Area in the Miocene Epoch " 114
" 21. The Volcanic Range of the Siebengebirge " 117
" 22. Section of Extinct Crater of the Roderberg " 120
" 23. Plan and Section of the Laacher See " 122
" 24. Extinct Craters in the Jaulan " 130
" 25. Mount Shasta " 139
" 26. Forms of Volcanic Tuff-Cones, Auckland " 148
" 27. "The White Rocks," Portrush, Co. Antrim " 157
" 28. Section across the Volcanic Plateau of Antrim " 159
" 29. Section at Templepatrick " 161
" 30. Cliff above the Giant's Causeway " 163
" 31. The Giant's Causeway, Co. Antrim " 165
" 32. "The Chimneys," North Coast of Antrim " 166
" 33. Section at Alt na Searmoin, Mull " 175
" 34. View of the Scuir of Eigg from the East " 181
Map of Volcanic Band of the Moluccas " 200
" 35. Map of the Krakatoa Group of Islands " 203
" 36. Section from Verlaten Island through Krakatoa " 204
Fig. 37. Isoseismals of the Charleston Earthquake Page 223
" 38. Photograph of the Moon's Surface " 241
" 39. Portion of the Moon's Surface " 243
PLATES.
I. & II. Magnified Sections of Vesuvian Lavas.
III. & IV. Magnified Sections of Volcanic Rocks.
Volcanoes: Past and Present.
PART I.
INTRODUCTION.
CHAPTER I.
HISTORIC NOTICES OF VOLCANIC ACTION.
There are no manifestations of the forces of Nature more calculated to inspire us with feelings of awe and admiration than volcanic eruptions preceded or accompanied, as they generally are, by earthquake shocks. Few agents have been so destructive in their effects; and to the real dangers which follow such terrestrial convulsions are to be added the feelings of uncertainty and revulsion which arise from the fact that the earth upon which we tread, and which we have been accustomed to regard as the emblem of stability, may become at any moment the agent of our destruction. It is, therefore, not surprising that the ancient Greeks, who, as well as the Romans, were close observers of the phenomena of Nature, should have investigated the causes of terrestrial disturbances, and should have come to some conclusions upon them in accordance with the light they possessed. These terrible forces presented to the Greeks, who clothed all the operations of Nature in poetic imagery and deified her forces, their poetical and mystical side; and as there was a deity for every natural force, so there was one for earthquakes and volcanoes. Vulcan, the deformed son of Juno (whose name bears so strange a resemblance to that of "the first artificer in iron" of the Bible, Tubal Cain), is condemned to pass his days under Mount Etna, fabricating the thunderbolts of Jove, and arms for the gods and great heroes of antiquity.
The Pythagoreans appear to have held the doctrine of a central fire (meson pyr) as the source of volcanic phenomena; and in the Dialogues of Plato allusion is made to a subterranean reservoir of lava, which, according to Simplicius, was in accordance with the doctrine of the Pythagoreans which Plato was recounting.[1] Thucydides clearly describes the effect of earthquakes upon coast-lines of the Grecian Archipelago, similar to that which took place in the case of the earthquake of Lisbon, the sea first retiring and afterwards inundating the shore. Pliny supposed that it was by earthquake avulsion that islands were naturally formed. Thus Sicily was torn from Italy, Cyprus from Syria, Euboea from Boeotia, and the rest; but this view was previously enunciated by Aristotle in his "Peri kosmou," where he states that earthquakes have torn to pieces many parts of the earth, while lands have been converted into sea, and that tracts once covered by the sea have been converted into dry land.
But the most philosophical views regarding terrestrial phenomena are those given by Ovid as having been held by Pythagoras (about B.C. 580). In the Metamorphoses his views regarding the interchange of land and sea, the effects of running water in eroding valleys, the growth of deltas, the effect of earthquakes in burying cities and diverting streams from their sources, are remarkable anticipations of doctrines now generally held.[2] But what most concerns us at present are his views regarding the changes which have come over volcanic mountains. In his day Vesuvius was dormant, but Etna was active; so his illustrations are drawn from the latter mountain; and in this connection he observes that volcanic vents shift their position. There was a time, he says, when Etna was not a burning mountain, and the time will come when it will cease to burn; whether it be that some caverns become closed up by the movements of the earth, or others opened, or whether the fuel is finally exhausted.[3] Strabo may be regarded as having originated the view, now generally held, that active volcanoes are safety-valves to the regions in which they are situated. Referring to the tradition recorded by Pliny, that Sicily was torn from Italy by an earthquake, he observes that the land near the sea in those parts was rarely shaken by earthquakes, since there are now orifices whereby fire and ignited matters and waters escape; but formerly, when the volcanoes of Etna, the Lipari Islands, Ischia, and others were closed up, the imprisoned fire and wind might have produced far more violent movements.[4]
The account of the first recorded eruption of Vesuvius has been graphically related by the younger Pliny in his two letters to Tacitus, to which I shall have occasion to refer further on.[5] These bring down the references to volcanic phenomena amongst ancient authors to the commencement of the Christian era; from all of which we may infer that the more enlightened philosophers of antiquity had a general idea that eruptions had their origin in a central fire within the interior of the earth, that volcanic mountains were liable to become dormant for long periods, and afterwards to break out into renewed activity, that there existed a connection between volcanic action and earthquakes, and that volcanoes are safety-valves for the regions around.
It is unnecessary that I should pursue the historical sketch further. Those who wish to know the views of writers of the Middle Ages will find them recorded by Sir Charles Lyell.[6] The long controversy carried on during the latter part of the eighteenth century between "Neptunists," led by Werner on the one side, and "Vulcanists," led by Hutton and Playfair on the other, regarding the origin of such rocks as granite and basalt, was finally brought to a close by the triumph of the "Vulcanists," who demonstrated that such rocks are the result of igneous fusion; and that in the cases of basalt and its congeners, they are being extruded from volcanic vents at the present day. The general principles for the classification of rocks as recognised in modern science may be regarded as having been finally established by James Hutton, of Edinburgh, in his Theory of the Earth,[7] while they were illustrated and defended by Professor Playfair in his work entitled, Illustrations of the Huttonian Theory of the Earth,[8] although other observers, such as Desmarest, Collini, and Guettard, had in other countries come to very clear views on this subject.
The following are some of the more important works on the phenomena of volcanoes and earthquakes published during the present century:—[9]
1. Poulett Scrope, F.R.S., Considerations on Volcanoes (1825). This work is dedicated to Lyell, his fellow-worker in the same department of science, and was undertaken, as he says, "in order to help to dispel that signal delusion as to the mode of action of the subtelluric forces with which the Elevation-Crater theory had mystified the geological world." The second edition was published in 1872.
2. This was followed by the admirable work, On the Extinct Volcanoes of Central France, published in 1826 (2nd edition, 1858), and is one of the most complete monographs on a special volcanic district ever written.
3. Dr. Samuel Hibbert, History of the Extinct Volcanoes of the Basin of Neuwied on the Lower Rhine (1832). Dr. Hibbert's work is one of remarkable merit, if we consider the time at which it was written. For not only does it give a clear and detailed account of the volcanic phenomena of the Eifel and the Lower Rhine, but it anticipates the principles upon which modern writers account for the formation of river valleys and other physical features; and in working out the physical history of the Rhine valley below Mainz, and its connection with the extinct volcanoes which are found on both banks of that river, he has taken very much the same line of reasoning which was some years afterwards adopted by Sir A. Ramsay when dealing with the same subject. It does not appear that the latter writer was aware of Dr. Hibbert's treatise.
4. Leopold von Buch, Description Physique des Iles Canaries (1825), translated from the original by C. Boulanger (1836); Geognostische Reise (Berlin, 1809), 2 vols.; and Reise durch Italien (1809). From a large number of writings on volcanoes by this distinguished traveller, whom Alexander von Humboldt calls "dem geistreichen Forscher der Natur," the above are selected as being the most important. That on the Canaries is accompanied by a large atlas, in which the volcanoes of Teneriffe, Palma, and Lancerote, with some others, are elaborately represented, and are considered to bear out the author's views regarding the formation of volcanic cones by elevation or upheaval. The works dealing with the volcanic phenomena of Central and Southern Italy are also written with the object, in part at least, of illustrating and supporting the same theoretical views; with these we have to deal in the next chapter.
5. Dr. Charles Daubeny, F.R.S., Description of Active and Extinct Volcanoes, of Earthquakes, and of Thermal Springs, with remarks on the causes of these phenomena, the character of their respective products, and their influence on the past and present condition of the globe (2nd edition, 1848). In this work the author gives detailed descriptions of almost all the known volcanic districts of the globe, and defends what is called "the chemical theory of volcanic action"—a theory at one time held by Sir Humphrey Davy.
6. Wolfgang Sartorius von Waltershausen, Der Aetna. This work possesses a melancholy interest from the fact that its distinguished author did not live to see its publication. Von Waltershausen, having spent several years in making an elaborate survey of Etna, produced an atlas containing numerous detailed maps, views, and drawings of this mountain and its surroundings, which were published at Weimar by Engelmann in 1858. A description in MS. to accompany the atlas was also prepared, but before it was printed, the author died, on the 16th October 1876. The MS. having been put into the hands of the late Professor Arnold von Lasaulx by the publisher of the atlas, it was subsequently brought out under the care of this distinguished petrologist, who was so fully fitted for an undertaking of this kind.
7. Sir Charles Lyell in his Principles of Geology[10] devotes several chapters to the consideration of volcanic phenomena, in which, being in harmony with the views of his friend, Poulett Scrope, he combats the "elevation theory" of Von Buch, as applied to the formation of volcanic mountains, holding that they are built up of ashes, stones, and scoriae blown out of the throat of the volcano and piled around the orifice in a conical form. Together with these materials are sheets of lava extruded in a molten condition from the sides or throat of the crater itself.
8. Professor J. W. Judd, F.R.S., in his able work entitled, Volcanoes: What they are, and what they teach,[11] has furnished the student of vulcanicity with a very complete manual of a general character on the subject. The author, having extensive personal acquaintance with the volcanoes of the south of Europe and the volcanic rocks of the British Isles, was well equipped for undertaking a work of the kind; and in it he supports the views of Lyell and Scrope regarding the mode of formation of volcanic mountains.
9. Sir Archibald Geikie, F.R.S., in his elaborate monograph[12] on the Tertiary Volcanic Rocks of the British Isles, has recorded his views regarding the origin and succession of the plateau basalts and associated rocks over the region extending from the north of Ireland to the Inner Hebrides; and in dealing with these districts in the following pages I have made extensive use of his observations and conclusions.
10. Report published by the Royal Society on the Eruption of Krakatoa—drawn up by several authors (1885)—and the work on the same subject by Chev. Verbeek, and published by the Government of the Netherlands (1886). In these works all the phenomena connected with the extraordinary eruptions of Krakatoa in 1883 are carefully noted and scientifically discussed, and illustrated by maps and drawings.
11. The Charleston Earthquake of August 31, 1886, by Captain Clarence Edward Dutton, U.S. Ordnance Corps. Ninth Annual Report of the United States Geological Survey, 1887-88, with maps and illustrations.
12. Amongst other works which may be consulted with advantage is that of Mr. T. Mellard Reade on The Origin of Mountain Ranges; the Rev. Osmond Fisher's Physics of the Earth; Professor G. H. Darwin and Mr. C. Davison on "The Internal Tension of the Earth's Crust," Philosophical Transactions of the Royal Society, vol. 178; Mr. R. Mallet, "On the Dynamics of Earthquakes," Trans. Roy. Irish Academy, vol. xxi.; Professor O'Reilly's "Catalogues of Earthquakes," Trans. Roy. Irish Academy, vol. xxviii. (1884 and 1888); and Mr. A. Ent. Gooch On the Causes of Volcanic Action (London, 1890). These and other authorities will be referred to in the text.
[1] See Julius Schwarez On the Failure of Geological Attempts made by the Greeks. (Edition 1888.)
[2] "Vidi ego, quod fuerat quondam solidissima tellus, Esse fretum. Vidi factas ex aequore terras: Et procul a pelago conchae jacuere marinae; Et vetus inventa est in montibus anchora sumnis. Quodque fuit campus, vallem de cursus aquarum Fecit; et eluvie mons est deductus in aequor: Eque paludosa siccis humus aret arenis; Quaeque sitim tulerant, stagnata paludibus hument. Hic fontes Natura novos emissit, at illuc Clausit: et antiquis concussa tremoribus orbis Fulmina prosiliunt...." —Lib. xv. 262.
[3] "Nec, quae sulfureis ardet fornacibus, Aetne Ignea semper erit; neque enim fuit ignea semper. Nam, sive est animal tellus, et vivit, habetque Spiramenta locis flammam exhalantia multis; Spirandi mutare vias, quotiesque movetur, Has finire potest, illas aperire cavernas: Sive leves imis venti cohibentur in antris; Saxaque cum saxis...." —Ibid., 340.
[4] Strabo, lib. vi.
[5] Tacitus, lib. vi. 16, 20.
[6] Principles of Geology, 11th edition, vol. i., ch. 3.
[7] 2 vols., Edin. (1795).
[8] Edin. (1802).
[9] A more extended list of early works will be found in Daubeny's Volcanoes (1848).
[10] 11th edition (1872).
[11] 4th edition (1888).
[12] "The History of Volcanic Action during the Tertiary Period in the British Isles," Trans. Roy. Soc., Edin. Vol. xxxv, (1888).
CHAPTER II.
FORM, STRUCTURE, AND COMPOSITION OF VOLCANIC MOUNTAINS.
The conical form of a volcanic mountain is so generally recognised, that many persons who have no intelligent acquaintance with geological phenomena are in the habit of attributing to all mountains having a conical form, and especially if accompanied by a truncated apex, a volcanic origin. Yet this is very far from being the fact, as some varieties of rock, such as quartzite, not unfrequently assume this shape. Of such we have an example in the case of Errigal, a quartzite mountain in Donegal, nearly 3000 feet high, which bears a very near approach in form to a perfect cone or pyramid, and yet is in no way connected, as regards its origin or structure, with volcanic phenomena. Another remarkable instance is that of Schehallion in Scotland, also composed of quartz-rock; and others may be found amongst the ranges of Islay and Jura, described by Sir A. Geikie.[1]
Notwithstanding, however, such exceptions, which might be greatly multiplied, the majority of cone-shaped mountains over the globe have a volcanic origin.[2] The origin of this form in each case is entirely distinct. In the case of quartzite mountains, the conical form is due to atmospheric influences acting on a rock of uniform composition, traversed by numerous joints and fissures crossing each other at obtuse angles, along which the rock breaks up and falls away, so that the sides are always covered by angular shingle forming slopes corresponding to the angle of friction of the rock in question. In the case of a volcanic mountain, however, the same form is due either to accumulation of fragmental material piled around the cup-shaped hollow, or crater, which is usually placed at the apex of the cone, and owing to which it is bluntly terminated, or else to the welling up from beneath of viscous matter in the manner presently to be described.
Views of Sir Humphrey Davy and L. von Buch.—The question how a volcanic cone came to be formed was not settled without a long controversy carried on by several naturalists of eminence. Some of the earlier writers of modern times on the subject of vulcanicity—such as Sir Humphrey Davy and Leopold von Buch—maintained that the conical form was due to upheaval by a force acting from below at a central focus, whereby the materials of which the mountain is formed were forced to assume a qua-qua versal position—that is, a position in which the materials dip away from the central focus in every direction. But this view, originally contested by Scrope and Lyell, has now been generally abandoned. It will be seen on reflection that if a series of strata of ashes, tuff, and lava, originally horizontal, or nearly so, were to be forced upwards into a conical form by a central force, the result would be the formation of a series of radiating fissures ever widening from the circumference towards the focus. In the case of a large mountain such fissures, whether filled with lava or otherwise, would be of great breadth towards the focus, or central crater, and could not fail to make manifest beyond dispute their mechanical origin. But no fissures of the kind here referred to are, as a matter of fact, to be observed. Those which do exist are too insignificant and too irregular in direction to be ascribed to such an origin; so that the views of Von Buch and Davy must be dismissed, as being unsupported by observation, and as untenable on dynamical grounds. As a matter of fact, the "elevatory theory," or the "elevation-crater theory," as it is called by Scrope, has been almost universally abandoned by writers on vulcanicity.
Principal Varieties of Volcanic Mountains as regards Form.—But whilst rejecting the "elevatory theory," it is necessary to bear in mind that volcanic cones and dome-shaped elevations have been formed in several distinct ways, giving rise to varieties of structure essentially different. Two of the more general of these varieties of form, the crater-cone and the dome, are found in some districts, as in Auvergne, side by side. The crater-cone consists of beds or sheets of ashes, lapilli, and slag piled up in a conical form, with a central crater (or cup) containing the principal pipe through which these materials have been erupted; the dome, of a variety of trachytic lava, which has been extruded in a molten, or viscous, condition from a central pipe, and in such cases there is no distinct crater. There are other forms of volcanic mountains, such as those built up of basaltic matter, of which I shall have to speak hereafter, but the two former varieties are the most prevalent; and we may now proceed to consider the conditions under which the crater-cone volcanoes have been formed.
Crateriform Volcanic Cones.—Of this class nearly all the active volcanoes of the Mediterranean region—Etna, Vesuvius, Stromboli, and the Lipari Islands—may be considered as representatives. They consist essentially of masses of fragmental material, which have from time to time been blown out of an orifice and piled up around with more or less regularity (according to the force exerted, and direction of the prevalent winds), alternating with sheets of lava. In this way mountains several thousand feet in height and of vast horizontal extent are formed. The fragmental materials thus accumulated are of all sizes, from the finest dust up to blocks many tons in weight, the latter being naturally piled around nearest to the orifice. The fine dust, blown high into the air by the explosive force of the gases and vapours, is often carried to great distances by the prevalent winds. Thus during the eruption of Vesuvius in A.D. 472 showers of ashes, carried high into the air by the westerly wind, fell over Constantinople at a distance of 750 miles.[3]
These loose, or partially consolidated, fragmental materials are rudely stratified, and slope downwards and outwards from the edge of the crater, so as to present the appearance of what is known as "the dip" of stratified deposits which have been upraised from the horizontal position by terrestrial forces. It was this excentrical arrangement which gave rise to the supposition that such volcanic ash-beds had been tilted up by a force acting in the direction of the volcanic throat, or orifice of eruption. The interior wall of Monte di Somma, the original crater of Vesuvius, presents a good illustration of such fragmental beds. I shall have occasion further on to describe more fully the structure of this remarkable mountain; so that it will suffice to say here that this old prehistoric crater, the walls of which enclose the modern cone of Vesuvius, is seen to be formed of irregular beds of ash, scoriae, and fragmental masses, traversed by numerous dykes of lava, and sloping away outwards towards the surrounding plains.
Of similar materials are the flanks of Etna composed, even at great distances from the central crater; the beds of ash and agglomerate sometimes alternating with sheets of solidified lava and traversed by dykes of similar material of later date, injected from below through fissures formed during periods of eruptive energy. Numerous similar examples are to be observed in the Auvergne region of Central France and the Eifel. And here we find remarkable cases of "breached cones," or craters, which will require some special description. Standing on the summit of the Puy de Dome, and looking northwards or southwards, the eye wanders over a tract formed of dome-shaped hills and of extinct crater-cones rising from a granitic platform. But what is most peculiar in the scene is the ruptured condition of a large number of the cones with craters. In such cases the wall of the crater has been broken down on one side, and we observe that a stream of lava has been poured out through the breach and overflowed the plain below. The cause of this breached form is sufficiently obvious. In such cases there has been an explosion of ashes, stones, and scoriae from the volcanic throat, by which a cone-shaped hill with a crater has been built up. This has been followed by molten lava welling up through the throat, and gradually filling the crater. But, as the lava is much more dense than the material of which the crater wall is composed, the pressure of the lava outwards has become too great for the resistance of the wall, which consequently has given way at its weakest part and, a breach being formed, the molten matter has flowed out in a stream which has inundated the country lying at the base of the cone. In one instance mentioned by Scrope, the original upper limit of the lake of molten lava has left its mark in the form of a ring of slag on the inside of the breached crater.[4]
Craterless Domes.—These differ essentially both in form and composition from those just described, and have their typical representatives in the Auvergne district, though not without their analogues elsewhere, as in the case of Chimborazo, in South America, one of the loftiest volcanic mountains in the world.
Taking the Puy de Dome, Petit Suchet, Cliersou, Grand Sarcoui in Auvergne, and the Mamelon in the Isle of Bourbon as illustrations, we have in all these cases a group of volcanic hills, dome-shaped and destitute of craters, the summits being rounded or slightly flattened. We also observe that the flanks rise more abruptly from their bases, and contrast in outline with the graceful curve of the crater cones. The dome-shaped volcanoes are generally composed of felsitic matter, whether domite, trachyte, or andesite, which has been extruded in a molten or viscous condition from some orifice or fissure in the earth's crust, and being piled up and spreading outwards, necessarily assumes such a form as that of a dome, as has been shown by experiment on a small scale by Dr. E. Reyer, of Graetz.[5] The contrast between the two forms (those of the dome and the crater-cone) is exemplified in the case of the Grand Sarcoui and its neighbours. The former is composed of a species of trachyte; the latter of ashes and fragmental matter which have been blown out of their respective vents of eruption into the air, and piled up and around in a crateriform manner with sides of gradually diminishing slope outwards, thus giving rise to the characteristic volcanic curve. The two varieties here referred to, contrasting in form, composition, and colour of material, can be clearly recognised from the summit of the Puy de Dome, which rises by a head and shoulders above its fellows, and thus affords an advantageous standpoint from which to compare the various forms of this remarkable group of volcanic mountains.
Cotopaxi (Fig. 2) has been generally supposed to be a dome; but Whymper, who ascended the mountain in 1880, shows that it is a cone with a crater, 2,300 feet in largest diameter. He determined the height to be 19,613 feet above the ocean. Its real elevation above the sea is somewhat masked, owing to the fact that it rises from the high plain of Tapia, which is itself 8,900 feet above the sea surface. The smaller peak on the right (Fig. 2) is that of Carihuairazo, which reaches an elevation of over 16,000 feet.
Chimborazo, in Columbia, province of Quito, is one of the loftiest of the chain of the Andes, and is situated in lat. 1 deg. 30' S., long. 78 deg. 58' W. Though not in a state of activity, it is wholly composed of volcanic material, and reaches an elevation of over 20,000 feet above the ocean; its sides being covered by a sheet of permanent snow to a level of 2,600 feet below the summit.[6] Seen from the shores of the Pacific, after the long rains of winter, it presents a magnificent spectacle, "when the transparency of the air is increased, and its enormous circular summit is seen projected upon the deep azure blue of the equatorial sky. The great rarity of the air through which the tops of the Andes are seen adds much to the splendour of the snow, and aids the magical effect of its reflection."
Chimborazo was ascended by Humboldt and Bonpland in 1802 almost to the summit; but at a height of 19,300 feet by barometrical measurement, their further ascent was arrested by a wide chasm. Boussingault, in company with Colonel Hall, accomplished the ascent as far as the foot of the mass of columnar "trachyte," the upper surface of which, covered by a dome of snow, forms the summit of the mountain. The whole mass of the mountain consists of volcanic rock, varieties of andesite; there is no trace of a crater, nor of any fragmental materials, such as are usually ejected from a volcanic vent of eruption.[7]
Lava Crater-Cones.—A third form of volcanic mountain is that which has been built up by successive eruptions of basic lava, such as basalt or dolerite, when in a molten condition. These are very rare, and the slope of the sides depends on the amount of original viscosity. Where the lava is highly fused its slope will be slight, but if in a viscous condition, successive outpourings from the orifice, unable to reach the base of the mountain, will tend to form a cone with increasing slope upwards. Mauna Loa and Kilauea, in the Hawaiian Group, according to Professor J. D. Dana, are basalt volcanoes in a normal state. They have distinct craters, and the material of which the mountain is formed is basalt or dolerite. The volcano of Rangitoto in Auckland, New Zealand, appears to belong to this class.
Basalt is the most fusible of volcanic rocks, owing to the augite and magnetite it contains, so that it spreads out with a very slight slope when highly fused. Trachyte, on the other hand, is the least fusible owing to the presence of orthoclase felspar, or quartz; so that the volcanic domes formed of this material stand at a higher angle from the horizon than those of basaltic cones.
[1] Scenery and Geology of Scotland (1865), p. 214.
[2] Humboldt says: "The form of isolated conical mountains, as those of Vesuvius, Etna, the Peak of Teneriffe, Tunguagua, and Cotopaxi, is certainly the shape most commonly observed in volcanoes all over the globe."—Views of Nature, translated by E. C. Otte and H. G. Bohn (1850).
[3] It is supposed that after the disastrous explosion of Krakatoa in 1883 the fine dust carried into the higher regions of the atmosphere was carried round almost the entire globe, and remained suspended for a lengthened period, as described in a future page.
[4] Another remarkable case is mentioned and figured by Judd, where one of the Lipari Isles, composed of pumice and rising out of the Mediterranean, has been breached by a lava-stream of obsidian.—Loc. cit., p. 123.
[5] Reyer has produced such dome-shaped masses by forcing a quantity of plaster of Paris in a pasty condition up through an orifice in a board; referred to by Judd, loc. cit., p. 125.
[6] Whymper determined the height to be 20,498 feet; Reiss and Stuebel make it 20,703 feet. Whymper thinks there may be a crater concealed beneath the dome of snow.—Travels amongst the Great Andes of the Equator, by Edward Whymper (1892).
[7] Whymper states that there is a prevalent idea that Cotopaxi and a volcano called Sangai act as safety-valves to each other. Sangai reaches an elevation (according to Reiss and Stuebel) of 17,464 feet, and sends intermittent jets of steam high into the air, spreading out into vast cumulus clouds, which float away southwards, and ultimately disappear.—Ibid., p. 73.
CHAPTER III.
LINES AND GROUPS OF ACTIVE VOLCANIC VENTS.
The globe is girdled by a chain of volcanic mountains in a state of greater or less activity, which may perhaps be considered a girdle of safety for the whole world, through which the masses of molten matter in a state of high pressure beneath the crust find a way of escape; and thus the structure of the globe is preserved from even greater convulsions than those which from time to time take place at various points on its surface. This girdle is partly terrestrial, partly submarine; and commencing at Mount Erebus, near the Antarctic Pole, ranging through South Shetland Isle, Cape Horn, the Andes of South America, the Isthmus of Panama, then through Central America and Mexico, and the Rocky Mountains to Kamtschatka, the Aleutian Islands, the Kuriles, the Japanese, the Philippines, New Guinea, and New Zealand, reaches the Antarctic Circle by the Balleny Islands. This girdle sends off branches at several points. (See Map, p. 23.)
(a.) The linear arrangement of active or dormant volcanic vents has been pointed out by Humboldt, Von Buch, Daubeny, and other writers. The great range of burning mountains of the Andes of Chili, Peru, Bolivia, and Mexico, that of the Aleutian Islands, of Kamtschatka and the Kurile Islands, extending southwards into the Philippines, and the branching range of the Sunda Islands are well-known examples. That of the West Indian Islands, ranging from Grenada through St. Vincent, St. Lucia, Martinique, Dominica, Guadeloupe, Montserrat, Nevis, and St. Eustace,[1] is also a remarkable example of the linear arrangement of volcanic mountains. On tracing these ranges on a map of the world[2] (Map, p. 23), it will be observed that they are either strings of islands, or lie in proximity to the ocean; and hence the view was naturally entertained by some writers that oceanic water, or at any rate that of a large lake or sea, was a necessary agent in the production of volcanic eruptions. This view seems to receive further corroboration from the fact that the interior portions of the continents and large islands such as Australia are destitute of volcanoes in action, with the remarkable exceptions of Mounts Kenia and Kilimanjaro in Central Africa, and a few others. It is also very significant in this connection that many of the volcanoes now extinct, or at least dormant, both in Europe and Asia, appear to have been in proximity to sheets of water during the period of activity. Thus the old volcanoes of the Hauran, east of the Jordan, appear to have been active at the period when the present Jordan valley was filled with water to such an extent as to constitute a lake two hundred miles in length, but which has now shrunk back to within the present limits of the Dead Sea.[3] Again, at the period when the extinct volcanoes of Central France were in active operation, an extensive lake overspread the tract lying to the east of the granitic plateau on which the craters and domes are planted, now constituting the rich and fertile plain of Clermont.
Such instances are too significant to allow us to doubt that water in some form is very generally connected with volcanic operations; but it does not follow that it was necessary to the original formation of volcanic vents, whether linear or sporadic. If this were so, the extinct volcanoes of the British Isles would still be active, as they are close to the sea-margin, and no volcano would now be active which is not near to some large sheet of water. But Jorullo, one of the great active volcanoes of Mexico, lies no less than 120 miles from the ocean, and Cotopaxi, in Ecuador, is nearly equally distant. Kilimanjaro, 18,881 feet high, and Kenia, in the equatorial regions of Central Africa, are about 150 miles from the Victoria Nyanza, and a still greater distance from the ocean; and Mount Demavend, in Persia, which rises to an elevation of 18,464 feet near the southern shore of the Caspian Sea, a volcanic mountain of the first magnitude, is now extinct or dormant.[4] Such facts as these all tend to show that although water may be an accessory of volcanic eruptions, it is not in all cases essential; and we are obliged, therefore, to have recourse to some other theory of volcanic action differing from that which would attribute it to the access of water to highly heated or molten matter within the crust of the earth.
(b.) Leopold von Buch on Rents and Fissures in the Earth's Crust.—The view of Leopold von Buch, who considered that the great lines of volcanic mountains above referred to rise along the borders of rents, or fissures, in the earth's crust, is one which is inherently probable, and is in keeping with observation. That the crust of the globe is to a remarkable extent fissured and torn in all directions is a phenomenon familiar to all field geologists. Such rents and fissures are often accompanied by displacement of the strata, owing to which the crust has been vertically elevated on one side or lowered on the other, and such displacements (or "faults") sometimes amount to thousands of feet. It is only occasionally, however, that such fractures are accompanied by the extrusion of molten matter; and in the North of England and Scotland dykes of igneous rock, such as basalt, which run across the country for many miles in nearly straight lines, often cut across the faults, and are only rarely coincident with them. Nevertheless, it can scarcely be a question that the grand chain of volcanic mountains which stretches almost continuously along the Andes of South America, and northwards through Mexico, has been piled up along the line of a system of fissures in the fundamental rocks parallel to the coast, though not actually coincident therewith.
(c.) The Cordilleras of Quito.—The structure and arrangement of the Cordilleras of Quito, for example, are eminently suggestive of arrangement along lines of fissure. As shown by Alexander von Humboldt,[5] the volcanic mountains are disposed in two parallel chains, which run side by side for a distance of over 500 miles northwards into the State of Columbia, and enclose between them the high plains of Quito and Lacunga. Along the eastern chain are the great cones of El Altar, rising to an elevation of 16,383 feet above the ocean, and having an enormous crater apparently dormant or extinct, and covered with snow; then Cotopaxi (Fig. 2), its sides covered with snow, and sending forth from its crater several columns of smoke; then Guamani and Cayambe (19,000 feet), huge truncated cones apparently extinct; these constitute the eastern chain of volcanic heights. The western chain contains even loftier mountains. Here we find the gigantic Chimborazo, an extinct volcano whose summit is white with snow; Carihuairazo[6] and Illiniza, a lofty pointed peak like the Matterhorn; Corazon, a snow-clad dome, reaching a height of 15,871 feet; Atacazo and Pichincha, the latter an extinct volcano reaching an elevation of 15,920 feet; such is the western chain, remarkable for its straightness, the volcanic cones being planted in one grand procession from south to north. This rectilinear arrangement of the western chain, only a little less conspicuous in the eastern, is very suggestive of a line of fracture in the crust beneath. And when we contemplate the prodigious quantity of matter included within the limits of these colossal domes and their environments, all of which has been extruded from the internal reservoirs, we gain some idea of the manner in which the contracting crust disposes of the matter it can no longer contain.[7]
Between the volcanoes of Quito and those of Peru there is an intervening space of fourteen degrees of latitude. This is occupied by the Andes, regarding the structure of which we have not much information except that at this part of its course it is not volcanic. But from Arequipa in Peru (lat. 16 deg. S.), an active volcano, we find a new series of volcanic mountains continued southwards through Tacora (19,740 feet), then further south the more or less active vents of Sajama (22,915 feet), Coquina, Tutupaca, Calama, Atacama, Toconado, and others, forming an almost continuous range with that part of the desert of Atacama pertaining to Chili. Through this country we find the volcanic range appearing at intervals; and still more to the southwards it is doubtless connected with the volcanoes of Patagonia, north of the Magellan Straits, and of Tierra del Fuego. Mr. David Forbes considers that this great range of volcanic mountains, lying nearly north and south, corresponds to a line of fracture lying somewhat to the east of the range.[8]
(d.) Other Volcanic Chains.—A similar statement in all probability applies to the systems of volcanic mountains of the Aleutian Isles, Kamtschatka, the Kuriles, the Philippines, and Sunda Isles. Nor can it be reasonably doubted that the western American coast-line has to a great extent been determined, or marked out, by such lines of displacement; for, as Darwin has shown, the whole western coast of South America, for a distance of between 2000 and 3000 miles south of the Equator, has undergone an upward movement in very recent times—that is, within the period of living marine shells—during which period the volcanoes have been in activity.[9]
(e.) The Kurile Islands.—This chain may also be cited in evidence of volcanic action along fissure lines. It connects the volcanoes of Kamtschatka with those of Japan, and the linear arrangement is apparent. In the former peninsula Erman counted no fewer than thirteen active volcanic mountains rising to heights of 12,000 to 15,000 feet above the sea.[10] In the chain of the Kuriles Professor John Milne counted fifty-two well-defined volcanoes, of which nine, perhaps more, are certainly active.[11] They are not so high as those of Kamtschatka; but, on the other hand, they rise from very deep oceanic waters, and have been probably built up from the sea bottom by successive eruptions of tuff, lava, and ash. According to the view of Professor Milne, the volcanoes of the Kurile chain are fast becoming extinct.
(f.) Volcanic Groups.—Besides the volcanic vents arranged in lines, of which we have treated above, there are a large number, both active and extinct, which appear to be disposed in groups, or sporadically distributed, over various portions of the earth's surface. I say appear to be, because this sporadic distribution may really be resolvable (at least in some cases) into linear distribution for short distances. Thus the Neapolitan Group, which might at first sight seem to be arranged round Vesuvius as a centre, really resolves itself into a line of active and extinct vents of eruption, ranging across Italy from the Tyrrhenian Sea to the Adriatic, through Ischia, Procida, Monte Nuovo and the Phlegraean Fields, Vesuvius, and Mount Vultur.[12] Again, the extinct volcanoes of Central France, which appear to form an isolated group, indicate, when viewed in detail, a linear arrangement ranging from north to south.[13] Another region over which extinct craters are distributed lies along the banks of the Rhine, above Bonn and the Moselle; a fourth in Hungary; a fifth in Asia Minor and Northern Palestine; and a sixth in Central Asia around Lake Balkash. These are all continental, and the linear distribution is not apparent.
[1] For an interesting account of this range of volcanic islands see Kingsley's At Last. The grandest volcanic peak is that of Guadeloupe, rising to a height of 5000 feet above the ocean, amidst a group of fourteen extinct craters. But the most active vent of the range is the Souffriere of St. Vincent. In the eruption of 1812 this mountain sent forth clouds of pumice, scoriae and ashes, some of which were carried by an upper counter current to Barbados, one hundred miles to the eastward, covering the surface with volcanic dust to a depth of several inches.
[2] An excellent, and perhaps the most recent, map of this kind is that given by Professor Prestwich in his Geology, vol. i. p. 216. One on a larger scale is that by Keith Johnston in his Physical Atlas.
[3] Memoir on the Physical Geology and Geography of Arabia Petraea, Palestine, etc., published for the Committee of the Palestine Exploration Fund (1886), p. 113, etc.
[4] This mountain was ascended in 1837 by Mr. Taylor Thomson, who found the summit covered with sulphur, and from a cone fumes at a high temperature issued forth, but there was no eruption.—Journ. Roy. Geographical Soc., vol. viii. p. 109.
[5] Humboldt, Atlas der Kleineren Schriften (1853).
[6] Ascended by Whymper June 29, 1880. He found the elevation to be 16,515 feet.
[7] The arrangement of the volcanoes of Peru and Bolivia is also suggestive of a double line of fissure, while those of Chili suggest one single line. The volcanoes of Arequipa, in the southern part of Peru, are dealt with by Dr. F. H. Hatch, in his inaugural dissertation, Ueber die Gesteine der Vulcan-Gruppe von Arequipa (Wien, 1886). The volcanoes rise to great elevations, having their summits capped by snow. The volcano of Charchani, lying to the north of Arequipa, reaches an elevation of 18,382 Parisian feet. That of Pichupichu reaches a height of 17,355 Par. feet. The central cone of Misti has been variously estimated to range from 17,240 to 19,000 Par. feet. The rocks of which the mountains are composed consist of varieties of andesite.
[8] D. Forbes, "On the Geology of Bolivia and Southern Peru," Quarterly Journal of the Geological Society, vol. xvii. p. 22 (1861).
[9] Darwin, Structure and Distribution of Coral Reefs, second edition, p. 186.
[10] Erman, Reise um die Welt.
[11] Milne, "Cruise amongst the Kurile Islands," Geol. Mag., New Ser. (August 1879).
[12] See Daubeny, Volcanoes, Map I.
[13] Sir A. Geikie has connected as a line of volcanic vents those of Sicily, Italy, Central France, the N. E. of Ireland, the Inner Hebrides and Iceland, of which the central vents are extinct or dormant, the extremities active.
CHAPTER IV.
MID-OCEAN VOLCANIC ISLANDS.
Oceanic Islands.—By far the most extensive regions with sporadically distributed volcanic vents, both active and extinct, are those which are overspread by the waters of the ocean, where the vents emerge in the form of islands. These are to be found in all the great oceans, the Atlantic, the Pacific, and the Indian; but are especially numerous over the central Pacific region. As Kotzebue and subsequently Darwin have pointed out, all the islands of the Pacific are either coral-reefs or of volcanic origin; and many of these rise from great depths; that is to say, from depths of 1000 to 2000 fathoms. It is unnecessary here to attempt to enumerate all these islands which rise in solitary grandeur from the surface of the ocean, and are the scenes of volcanic operations; a few may, however, be enumerated.
(a.) Iceland.—In the Atlantic, Iceland first claims notice, owing to the magnitude and number of its active vents and the variety of the accompanying phenomena, especially the geysers. As Lyell has observed,[1] with the exception of Etna and Vesuvius, the most complete chronological records of a series of eruptions in existence are those of Iceland, which come down from the ninth century of our era, and which go to show that since the twelfth century there has never been an interval of more than forty years without either an eruption or a great earthquake. So intense is the volcanic energy in this island that some of the eruptions of Hecla have lasted six years without cessation. Earthquakes have often shaken the whole island at once, causing great changes in the interior, such as the sinking down of hills, the rending of mountains, the desertion by rivers of their channels, and the appearance of new lakes. New islands have often been thrown up near the coast, while others have disappeared. In the intervals between the eruptions, innumerable hot springs afford vent to the subterranean heat, and solfataras discharge copious streams of inflammable matter. The volcanoes in different parts of the island are observed, like those of the Phlegraean Fields, to be in activity by turns, one vent serving for a time as a safety-valve for the others. The most memorable eruption of recent years was that of Skaptar Jokul in 1783, when a new island was thrown up, and two torrents of lava issued forth, one 45 and the other 50 miles in length, and which, according to the estimate of Professor Bischoff, contained matter surpassing in magnitude the bulk of Mont Blanc. One of these streams filled up a large lake, and, entering the channel of the Skapta, completely dried up the river. The volcanoes of Iceland may be considered as safety-valves to the region in which lie the British Isles.
(b.) The Azores, Canary, and Cape de Verde Groups.—This group of volcanic isles rises from deep Atlantic waters north of the Equator, and the vents of eruption are partially active, partially dormant, or extinct. It must be supposed, however, that at a former period volcanic action was vastly more energetic than at present; for, except at the Grand Canary, Gomera, Forta Ventura, and Lancerote, where various non-volcanic rocks are found, these islands appear to have been built up from their foundations of eruptive materials. The highest point in the Azores is the Peak of Pico, which rises to a height of 7016 feet above the ocean. But this great elevation is surpassed by that of the Peak of Teneriffe (or Pic de Teyde) in the Canaries, which attains to an elevation of 12,225 feet, as determined by Professor Piazzi Smyth.[2]
This great volcanic cone, rising from the ocean, its summit shrouded in snow, and often protruding above the clouds, must be an object of uncommon beauty and interest when seen from the deck of a ship. (Fig. 4.) The central cone, formed of trachyte, pumice, obsidian, and ashes, rises out of a vast caldron of older basaltic rocks with precipitous inner walls—much as the cone of Vesuvius rises from within the partially encircling walls of Somma. (Fig. 5.) From the summit issue forth sulphurous vapours, but no flame.
Piazzi Smyth, who during a prolonged visit to this mountain in 1856 made a careful survey of its form and structure, shows that the great cone is surrounded by an outer ring of basalt enclosing two foci of eruption, the lavas from which have broken through the ring of the outer crater on the western side, and have poured down the mountain. At the top of the peak its once active crater is filled up, and we find a convex surface ("The Plain of Rambleta") surmounted towards its eastern end by a diminutive cone, 500 feet high, called "Humboldt's Ash Cone." The slope of the great cone of Teneriffe ranges from 28 deg. to 38 deg.; and below a level of 7000 feet the general slope of the whole mountain down to the water's edge varies from 10 deg. to 12 deg. from the horizontal. The great cone is penetrated by numerous basaltic dykes.
The Cape de Verde Islands, which contain beds of limestone along with volcanic matter, possess in the island of Fuego an active volcano, rising to a height of 7000 feet above the surface of the ocean. The central cone, like that of Teneriffe, rises from within an outer crater, formed of basalt alternating with beds of agglomerate, and traversed by numerous dykes of lava. This has been broken down on one side like that of Somma; and over its flanks are scattered numerous cones of scoriae, the most recent dating from the years 1785 and 1799.[3]
The volcanoes of Lancerote have a remarkably linear arrangement from west to east across the island. They are not yet extinct; for an eruption in 1730 destroyed a large number of villages, and covered with lava the most fertile tracts in the island, which at the time of Leopold von Buch's visit lay waste and destitute of herbage.[4] In the island of Palma there is one large central crater, the Caldera de Palma, three leagues in diameter, the walls of which conform closely to the margin of the coast. Von Buch calls this crater "une merveille de la nature," for it distinguishes this isle from all the others, and renders it one of the most interesting and remarkable amongst the volcanic islands of the ocean. The outer walls are formed of basaltic sheets, and towards the south this great natural theatre is connected with the ocean by a long straight valley, called the "Barranco de los Dolores," along whose sides the structure of the mountain is deeply laid open to view. The outer flanks of the crater are furrowed by a great number of smaller barrancos radiating outward from the rim of the caldera. Von Buch regards the barrancos as having been formed during the upheaval of the island, according to his theory of the formation of such mountains (the elevation-theory); but unfortunately for his views, these ravines widen outwards from the centre, or at least do not become narrower in that direction, as would be the case were the elevation-theory sound. The maps which accompany Von Buch's work are remarkably good, and were partly constructed by himself.
(c.) Volcanic Islands in the Atlantic south of the Equator.—The island of Ascension, formed entirely of volcanic matter, rises from a depth of 2000 fathoms in the very centre of the Atlantic. As described by Darwin, the central and more elevated portions are formed of trachytic matter, with obsidian and laminated ash beds. Amongst these are found ejected masses of unchanged granite, fragments of which have been torn from the central pipe during periods of activity, and would seem to indicate a granitic floor, or at least an original floor upon which more recent deposits may have been superimposed. In St. Helena we seem, according to Daubeny, to have the mere wreck of one great crater, no one stream of lava being traceable to its source, while dykes of lava are scattered in profusion throughout the whole substance of the basaltic masses which compose the island. Tristan da Cunha, in the centre of the South Atlantic, rises abruptly from a depth of 12,150 feet, at a distance of 1500 miles from any land; and one of its summits reaches an elevation of 7000 feet, being a truncated cone composed of alternating strata of tuff and augitic lava, surrounding a crater in which is a lake of pure water. The volcano is extinct or dormant.
Were the waters of the ocean to be drawn off, these volcanic islands would appear like stupendous conical mountains, far loftier, and with sides more precipitous, than any to be found on our continental lands, all of which rise from platforms of considerable elevation. The enormous pressure of the water on their sides enables these mid-oceanic islands to stand with slopes varying from the perpendicular to a smaller extent than if they were sub-aerial; and it is on this account that we find them rising with such extraordinary abruptness from the "vasty deep."
(d.) Volcanic Islands of the Pacific.—The volcanic islands of this great ocean are scattered over a wide tract on both sides of the equator. Those to the north of this line include the Sandwich Islands, the Mariana or Ladrone Islands, South Island, and Bonin Sima; south of the equator, the Galapagos, New Britain, Salomon, Santa Cruz, New Hebrides, the Friendly and Society Isles. While the coral reefs and islands of the Pacific may be recognised by their slight elevation above the surface of the waters, those of volcanic origin and containing active or extinct craters of eruption generally rise into lofty elevations, so that the two kinds are called the Low Islands and High Islands respectively. Amongst the group are trachytic domes such as the Mountain of Tobreonu in the Society Islands, rising to a height probably not inferior to that of Etna, with extremely steep sides, and holding a lake on its summit.[5] The linear arrangement of some of the volcanic islands of the Pacific is illustrated by those of the Tonga, or Friendly, Group, lying to the north of New Zealand. They consist of three divisions—(1) the volcanic; (2) those formed of stratified volcanic tuff, sometimes entirely or partially covered by coralline limestone; and (3) those which are purely coralline. The first form a chain of lofty cones and craters, lying in a E.N.E. and W.S.W. direction, and rising from depths of over 1000 fathoms. Mr. J. J. Lister, who has described the physical characters of these islands, has shown very clearly that they lie along a line—probably that of a great fissure—stretching from the volcanic island of Amargura on the north (lat. 18 deg. S.), through Lette, Metis, Kao (3030 feet), Tofua, Falcon, Honga Tonga, and the Kermadec Group into the New Zealand chain on the south. Some of these volcanoes are in a state of intermittent activity, as in the case of Tofua (lat. 20 deg. 30' S.), Metis Island, and Amargura; the others are dormant or extinct. The whole group appears to have been elevated at a recent period, as some of the beds of coral have been raised 1272 feet and upward above the sea-level, as in the case of Eua Island.[6] The greater number of the Pacific volcanoes appear to be basaltic; such as those of the Hawaiian Group, which have been so fully described by Professor J. D. Dana.[7] Here fifteen volcanoes of the first class have been in brilliant action; all of which, except three, are now extinct, and these are in Hawaii the largest and most eastern of the group. This island contains five volcanic mountains, of which Kea, 13,805 feet, is the highest; next to that, Loa, 13,675 feet; after these, Hualalai, rising 8273 feet; Kilauea, 4158 feet; and Kohala, 5505 feet above the sea; this last is largely buried beneath the lavas of Mauna Kea. The group contains a double line of volcanoes, one lying to the north and west of the other; and as the highest of the Hawaiian Group rises from a depth in the ocean of over 2000 fathoms, the total elevation of this mountain from its base on the bed of the ocean is not far from 26,000 feet, an elevation about that of the Himalayas. Mauna Kea has long been extinct, Hualalai has been dormant since 1801; but Mauna Loa is terribly active, there having been several eruptions, accompanied by earthquakes, within recent years, the most memorable being those of 1852 and 1868. In the former case the lava rose from the deep crater into "a lofty mountain," as described by Mr. Coan,[8] and then flowed away eastward for a distance of twenty miles. The interior of the crater consists of a vast caldron, surrounded by a precipice 200 to 400 feet in depth, with a circumference of about fifteen miles, and containing within it a second crater, bounded by a black ledge with a steep wall of basalt—a crater within a crater; and from the floor of the inner crater, formed of molten basalt, in a seething and boiling state, arise a large number of small cones and pyramids of lava, some emitting columns of grey smoke, others brilliant flames and streams of molten lava, presenting a wonderful spectacle, the effect of which is heightened by the constant roaring of the vast furnaces below.[9]
[1] Principles of Geology, 11th edition, vol. ii. p. 48.
[2] Smyth, Report on the Teneriffe Astronomical Experiment of 1856. Humboldt makes the elevation 12,090 feet. A beautiful model of the Peak was constructed by Mr. J. Nasmyth from Piazzi Smyth's plans, of which photographs are given by the latter.
[3] Daubeny, loc. cit., p. 460.
[4] Iles Canaries, p. 37.
[5] Daubeny, loc. cit., p. 426.
[6] Lister, "Notes on the Geology of the Tonga Islands," Quart. Jour. Geol. Soc., No. 188, p. 590 (1891).
[7] Dana, Characteristics of Volcanoes, with Contributions of Facts and Principles from the Hawaiian Islands. London, 1890.—Also, Geology of the American Exploring Expedition—Volcanoes of the Sandwich Islands.
[8] Coan, Amer. Jour. of Science, 1853.
[9] W. Ellis, the missionary, has given a vivid description of this volcano in his Tour of Hawaii. London, 1826.—Plans of the crater will be found in Professor Dana's work above quoted.
PART II.
EUROPEAN VOLCANOES.
CHAPTER I.
VESUVIUS.
Having now dealt in a necessarily cursory manner with volcanoes of distant parts of the globe, we may proceed to the consideration of the group of active volcanoes which still survive in Europe, as they possess a special interest, not only from their proximity and facility of access, at least to residents in Europe and the British Isles, but from their historic incidents; and in this respect Vesuvius, though not by any means the largest of the group, stands the first, and demands more special notice. The whole group rises from the shores of the Mediterranean, and consists of Vesuvius, Etna, Stromboli, one of the Lipari Islands, and Vulcano, a mountain which has given the name to all mountains of similar origin with itself.[1] Along with these are innumerable cones and craters of extinct or dormant volcanoes, of which a large number have been thrown out on the flanks of Etna.
(a.) Prehistoric Ideas regarding the Nature of this Mountain.—Down to the commencement of the Christian era this mountain had given no ostensible indication that it contained within itself a powerful focus of volcanic energy. True, that some vague tradition that the mountain once gave forth fire hovered around its borders; and several ancient writers, amongst them Diodorus Siculus and Strabo, inferred from the appearances of the higher parts of the mountain and the character of the rocks, which were "cindery and as if eaten by fire," that the country was once in a burning state, "being full of fiery abysses, though now extinct from want of fuel." Seneca (B.C. 1 to A.D. 64) had detected the true character of Vesuvius, as "having been a channel for the internal fire, but not its food;" nevertheless, at this period the flanks of the mountain were covered by fields and vineyards, while the summit, partially enclosed with precipitous walls of the long extinct volcano, Somma, was formed of slaggy and scoriaceous material, with probably a covering of scrub. Here it was that the gladiator Spartacus (B.C. 72), stung by the intolerable evils of the Roman Government, retreated to the very summit of the mountain with some trusty followers. Clodius the Praetor, according to the narration of Plutarch, with a party of three thousand men, was sent against them, and besieged them in a mountain (meaning Vesuvius or Somma) having but one narrow and difficult passage, which Clodius kept guarded; all the rest was encompassed with broken and slippery precipices, but upon the top grew a great many wild vines; the besieged cut down as many as they had need of, and twisted them into ladders long enough to reach from thence to the bottom, by which, without any danger, all got down except one, who stayed behind to throw them their arms, after which he saved himself with the rest.[2] "On the top" must (as Professor Phillips observes) be interpreted the summit of the exterior slope or crater edge, which would appear from the narrative to have broken down on one side, affording an entrance and mode of egress by which Spartacus fell upon, and surprised, the negligent Clodius Glabrus.
In fancied security, villas, temples, and cities had been erected on the slopes of the mountain. Herculaneum, Pompeii, and Stabiae, the abodes of art, luxury, and vice, had sprung up in happy ignorance that they "stood on a volcano," and that their prosperity was to have a sudden and disastrous close.[3]
(b.) Premonitory Earthquake Shocks.—The first monitions of the impending catastrophe occurred in the 63rd year after Christ, when the whole Campagna was shaken by an earthquake, which did much damage to the towns and villas surrounding the mountain even beyond Naples. This was followed by other shocks; and in Pompeii the temple of Isis was so much damaged as to require reconstruction, which was undertaken and carried out by a citizen at his own expense.[4] These earthquake shakings continued for sixteen years. At length, on the night of August 24th, A.D. 79, they became so violent that the whole region seemed to reel and totter, and all things appeared to be threatened with destruction. The next day, about one in the afternoon, there was seen to rise in the direction of Vesuvius a dense cloud, which, after ascending from the summit of the mountain into the air for a certain height in one narrow, vertical trunk, spread itself out laterally in such a form that the upper part might be compared to the cluster of branches, and the lower to the stem of the pine which forms so common a feature in the Italian landscape.[5]
(c.) Pliny's Letters to Tacitus.—For an account of what followed we are indebted to the admirable letters of the younger Pliny, addressed to the historian Tacitus, recounting the events which caused, or accompanied, the death of his uncle, the elder Pliny, who at the time of this first eruption of Vesuvius was in command of the Roman fleet at the entrance to the Bay of Naples. These letters, which are models of style and of accurate description, are too long to be inserted here; but he recounts how the dense cloud which hung over the mountain spread over the whole surrounding region, sometimes illuminated by flashes of light more vivid than lightning; how showers of cinders, stones, and ashes fell in such quantity that his uncle had to flee from Stabiae, and that even at so great a distance as Misenum they encumbered the surface of the ground; how the ground heaved and the bed of the sea was upraised; how the cloud descended on Misenum, and even the island of Capreae was concealed from view; and finally, how, urged by a friend who had arrived from Spain, he, with filial affection, supported the steps of his mother in flying from the city of destruction. Such being the condition of the atmosphere and the effects of the eruption at a point so distant as Cape Misenum, some sixteen geographical miles from the focus of eruption, it is only to be expected that places not half the distance, such as Herculaneum, Pompeii, and even Stabiae, with many villages and dwellings, should have shared a worse fate. The first of these cities, situated on the coast of the Bay of Naples, appears to have been overwhelmed by volcanic mud; Pompeii was buried in ashes and lapilli, and Stabiae probably shared a similar fate.[6]
(d.) Appearance of the Mountain at the Commencement of the Christian Era.—At the time of the first recorded eruption Vesuvius appears to have consisted of only a single cone with a crater, now known as Monte di Somma, the central cone of eruption which now rises from within this outer ruptured casing not having been formed. (Fig. 6.) The first effect of the eruption of the year 79 was to blow out the solidified covering of slag and scoriae forming the floor of the caldron. Doubtless at the close of the eruption a cone of fragmental matter and lava of some slight elevation was built up, and, if so, was subsequently destroyed; for, as we shall presently see by the testimony of the Abate Guilio Cesare Braccini, who examined the mountain not long before the great eruption of A.D. 1631, there was no central cone to the mountain at that time; and the mountain had assumed pretty much the appearance it had at the time that Spartacus took refuge within the walls of the great crater.
(e.) Destruction of Pompeii.—Pompeii was overwhelmed with dry ashes and lapilli. Sir W. Hamilton found some of the stones to weigh eight pounds. At the time of the author's visit, early in April 1872, the excavations had laid open a section about ten feet deep, chiefly composed of alternating layers of small pumice stones (lapilli) and volcanic dust. It was during the sinking of a well in 1713 upon the theatre containing the statues of Hercules and Cleopatra that the existence of the ancient city was accidentally discovered.
(f.) More recent eruptions.—Since the first recorded eruption in A.D. 79 down to the present day, Vesuvius has been the scene of numerous intermittent eruptions, of which some have been recorded; but many, doubtless, are forgotten.
In A.D. 203, during the reign of Severus, an eruption of extraordinary violence took place, which is related by Dion Cassius, from whose narrative we may gather that at this time there was only one large crater, and that the central cone of Vesuvius had not as yet been upraised. In A.D. 472 an eruption occurred of such magnitude as to cover all Europe with fine dust, and spread alarm even at Constantinople.
(g.) Eruption of 1631.—In December 1631 occurred the great convulsion whose memorials are written widely on the western face of Vesuvius in ruined villages. This eruption left layers of ashes over hundreds of miles of country, or heaps of mud swept down by hot water floods from the crater; the crater itself having been dissipated in the convulsion. Braccini, who examined the mountain not long before this eruption, found apparently no cone (or mount) like that of the modern Vesuvius. He states that the crater was five miles in circumference, about a thousand paces deep (or in sloping descent), and its sides covered with forest trees and brushwood, while at the bottom there was a plain on which cattle grazed.[7] It would seem that the mountain had at this time enjoyed a long interval of rest, and that it had reverted to very much the same state in which it was at the period of the first eruption, when the flanks were peopled by inhabitants living in fancied security. But six months of violent earthquakes, which grew more violent towards the close of 1631, heralded the eruption which took place in December, accompanied by terrific noises from within the interior of the mountain. The inhabitants of the coast were thus warned of the approaching danger, and had several days to arrange for their safety; but in the end, a great part of Torre del Greco was destroyed, and a like fate overtook Resina and Granatello, with a loss of life reported at 18,000 persons. During the eruption clouds condensed into tempests of rain, and hot water from the mountain, forming deluges of mud, swept down the sides, and reached even to Nola and the Apennines. Nor was the sea unmoved. It retired during the violent earthquakes, and then returned full thirty paces beyond its former limits.
Not indeed until near the close of the seventeenth century is there any evidence that the central cone of Vesuvius was in existence; but in October 1685 an eruption occurred which is recorded by Sorrentino, during which was erected "a new mountain within, and higher than the old one, and visible from Naples," a statement evidently referable to the existing cone—so that it is little more than two centuries since this famous volcanic mountain assumed its present form.
(h.) Eruptions between the years 1500 and 1800.—Since A.D. 1500 there have been fifty-six recorded eruptions of Vesuvius; one of these in 1767 was of terrific violence and destructiveness, and is represented by Sir William Hamilton in views taken both before and during the eruption. A pen-and-ink drawing of the appearance of the crater before the eruption is here reproduced from Hamilton's picture, from which it will be seen that the central crater contained within itself a second crater-cone, from whence steam, lava, and stones were being erupted (Fig. 7). Thus it will be seen that Vesuvius at this epoch consisted of three crater-cones within each other. The first, Monte di Somma; the second, the cone of Vesuvius; and the third, the little crater-cone within the second. During this eruption, vast lava-sheets invaded the fields and vineyards on the flanks of the mountain. A vivid account of this eruption, as witnessed by Padre Torre, is given by Professor Phillips.[8] We shall pass over others without further reference until we come down to our own times, in which Vesuvius has resumed its old character, and in one grand exhibition of volcanic energy, which took place in 1872, has evinced to the world that it still contains within its deep-seated laboratory all the elements of destructive force which it exhibited at the commencement of our era.
(i.) Structure of the Neapolitan Campagna.—But before giving a description of this terrific outburst of volcanic energy, it may be desirable to give some account of the physical position and structure of this mountain, by which the phenomena of the eruption will be better understood. Vesuvius and the Neapolitan Campagna are formed of volcanic materials bounded on the west by the Gulf of Naples, and on the east and south by ranges of Jurassic limestone, a prolongation of the Apennines, which send out a spur bounding the bay on the south, and forming the promontory of Sorrento. The little island of Capri is also formed of limestone, and is dissevered from the promontory by a narrow channel. The northern side of the bay is, however, formed of volcanic materials; it includes the Phlegraean Fields (Campi Phlegraei), and terminates in the promontory of Miseno. Lying in the same direction are the islands of Procida and Ischia, also volcanic. Hence it will be seen that the two horns of the bay are formed of entirely different materials, that of Miseno on the north being volcanic, that of Sorrento on the south being composed of Jurassic limestone, of an age vastly more ancient than the volcanic rocks on the opposite shore. (Map, p. 52.)
The general composition of the Neapolitan Campagna, from which the mountain rises, has been revealed by means of the Artesian well sunk to a depth of about 500 metres (1640 feet) at the Royal Palace of Naples, and may be generalised as follows:—
{ Recent beds of volcanic tuff (1) From surface to depth of { with marine shells, and containing 715 feet { fragments of trachytic { lava, etc. (Volcanic Beds).
{ Bituminous sands and marls (2) From 715 to 1420 { with marine shells of recent { species(?) (Pre-Volcanic Beds).
(3) From 1420 to 1574 { EOCENE BEDS. Micaceous sandstone { and marl (Macigno).
(4) From 1574 to bottom { JURASSIC BEDS. Apennine { Limestone.
From the above section, for which we are indebted to Mr. Johnston-Lavis, the most recent writer on Vesuvius, it would appear that the first volcanic explosions by which the mountain was ultimately to be built up took place after the deposition of the sands and marls (No. 2), while the whole Campagna was submerged under the waters of the Mediterranean. By the accumulation of the stratified tuff (No. 1), the sea-bed was gradually filled up during a period of volcanic activity, and afterwards elevated into dry land.[9]
(j.) Present Form and Structure of Vesuvius and Somma.—The outer cone of Vesuvius, or Monte di Somma, rises from a circular platform by a moderately gentle ascent, and along the north and east terminates in a craggy crest, with a precipitous cliff descending into the Atria del Cavallo, forming the wall of the ancient crater throughout half its circumference; this wall is formed of scoriae, ashes, and lapilli, and is traversed by numerous dykes of lava. Along the west and south this old crater has been broken down; but near the centre there remains a round-backed ridge of similar materials, once doubtless a part of the original crater of Somma, rising above the slopes of lava on either hand. On this has been erected the Royal Observatory, under the superintendence of Professor Luigi Palmieri, where continuous observations are being made, by means of delicate seismometers, of the earth-tremors which precede or accompany eruptions; for it is only justice to say that Vesuvius gives fair warning of impending mischief, and the instruments are quick to notify any premonitory symptoms of a coming catastrophe. The elevation of the Observatory is 2080 feet above the sea.
On either side of the Observatory ridge are wide channels filled to a certain height with lavas of the nineteenth and preceding centuries, the most recent presenting an aspect which can only be compared to a confused multitude of black serpents and pachyderms writhing and interlocked in some frightful death-struggle. Some of this lava, ten years old, as we cross its rugged and black surface presents gaping fissures, showing the mass to be red-hot a few feet from the surface, so slow is the process of cooling. These lava-streams—some of them reaching to the sea-coast—have issued forth from the Atria at successive periods of eruption.
From the midst of the Atria rises the central cone, formed of cinders, scoriae, and lava-streams, and fissured along lines radiating from the axis. This cone is very steep, the angle being about 40 deg.-45 deg. from the horizontal, and is formed of loose cindery matter which gives way at every step, and is rather difficult to climb. But on reaching the summit we look down into the crater, displaying a scene of ever-varying characters, rather oval in form, and about 1100 yards in diameter. From the map of Professor Guiscardi, published in 1855, there are seen two minor craters within the central one, formed in 1850, and an outflow of lava from the N.W. down the cone. At the time of the author's visit the crater was giving indications, by the great quantity of sulphurous gas and vapour rising from its surface, and small jets of molten lava beginning to flow down the outer side, of the grand outburst of internal forces which was presently to follow.
(k.) Eruption of 1872.—The grand eruption of 1872, of which a detailed account is given by Professor Palmieri,[10] commenced with a slight discharge of incandescent projectiles from the crater; and on the 13th January an aperture appeared on the upper edge of the cone from which at first a little lava issued forth, followed by the uprising of a cone which threw out projectiles accompanied by smoke, whilst the central crater continued to detonate more loudly and frequently. This little cone ultimately increased in size, until in April it filled the whole crater and rose four or five metres above the brim. At this time abundant lavas poured down from the base of the cone into the Atria del Cavallo, thence turned into the Fossa della Vetraria in the direction of the Observatory and towards the Crocella, where they accumulated to such an extent as to cover the hillside for a distance of about 300 metres; then turning below the Canteroni, formed a hillock without spreading much farther.
In October another small crater was formed by the falling in of the lava, which after a few days gave vent to smoke and several jets of lava; and towards the end of October the detonations increased, the smoke from the central crater issued forth more densely mixed with ashes, and the seismographical apparatus was much disturbed. On the 3rd and 4th November copious and splendid lava-streams coursed down the principal cone on its western side, but were soon exhausted; and in the beginning of 1872 the little cone, regaining vigour, began to discharge lava from the summit instead of the base as heretofore.
In the month of March 1873, with the full moon, the cone opened on the north-west side—the cleavage being indicated by a line of fumaroles—and lava issued from the base and poured down into the Atria as far as the precipices of Monte di Somma. On the 23rd April (another full moon) the activity of the craters increased, and on the evening of the 24th splendid lava-streams descended the cone in various directions, attracting on the same night the visits of a great many strangers. A lamentable event followed on the 26th. A party of visitors, accompanied by inexperienced guides, and contrary to the advice of Professor Palmieri, insisted on ascending to the place from which the lava issued. At half-past three on the morning of the 26th they were in the Atria del Cavallo, when the Vesuvian cone was rent in a north-west direction and a copious torrent of lava issued forth. Two large craters formed at the summit of the mountain, discharging incandescent projectiles and ashes. A cloud of smoke enveloped the unhappy visitors, who were under a hail-storm of burning projectiles. Eight were buried beneath it, or in the lava, while eleven were grievously injured.[11] The lava-stream, flowing over that of 1871 in the Atria, divided into two branches, the smaller one flowing towards Resina, but stopping before reaching the town; the larger precipitated itself into the Fossa della Vetraria, occupying the whole width of 800 metres, and traversing the entire length of 1300 metres in three hours. It dashed into the Fossa di Farone, and reached the villages of Massa and St. Sebastiano, covering a portion of the houses, and, continuing its course through an artificial foss, or trench, invaded cultivated ground and several villages. If it had not greatly slackened after midnight, from failure of supply at its source, it would have reached Naples by Ponticelli and flowed into the sea. The eruption towards the end of April had reached its height. The Observatory ridge was bounded on either side by two fiery streams, which rendered the heat intolerable. Simultaneously with the opening of the great fissure two large craters opened at the summit, discharging with a dreadful noise an immense cloud of smoke and ashes, with bombs which rose to a height of 1300 metres above the brim of the volcano.[12] The torrents of fire which threatened Resina, Bosco, and Torre Annunziata, and which devastated the fertile country of Novelle, Massa, St. Sebastiano, and Cerole, and two partially buried cities, the continual thunderings and growling of the craters, caused such terror, that numbers abandoned their dwellings, flying for refuge into Naples, while many Neapolitans went to Rome or other places. Fortunately, the paroxysm had now passed, the lava-streams stopped in their course, and the great torrent which passed the shoulders of the Observatory through the Fossa della Vetraria lowered the level of its surface below that of its sides, which appeared like two parallel ramparts above it. Had these streams continued to flow on the 27th of April as they had done on the previous night, they would have reached the sea, bringing destruction to the very walls of Naples. During this eruption Torre del Greco was upraised to the extent of two metres, and nearly all the houses were knocked down.
The igneous period of eruption having terminated, the ashes, lapilli, and projectiles became more abundant, accompanied by thunder and lightning. On the 28th they darkened the air, and the terrific noise of the mountain continuing or increasing, the terror at Resina, Portici, and Naples became universal. It seemed as though the tragic calamities of the eruption of A.D. 79 were about to be repeated. But gradually the force of the explosions decreased, and the noise from the crater became discontinuous, so that on the 30th the detonations were very few, and by the 1st May the eruption was completely over.
Such is a condensed account of one of the most formidable eruptions of our era. In the frontispiece of this volume a representation, taken (by permission) from a photograph by Negretti & Zambra, is given, showing the appearance of Vesuvius during the final stage of the eruption, when prodigious masses of smoke, steam, and illuminated gas issued forth from the summit and overspread the whole country around with a canopy which the light of the sun could scarcely penetrate.
It will be noticed in the above account that, concurrently with the full moon, there were two distinct and special outbreaks of activity; one occurring in March, the other in the month following. That the conditions of lunar and solar attraction should have a marked effect on a part of the earth's crust, while under the tension of eruptive forces, is only what might be expected. At full moon the earth is between the sun and the moon, and at new moon the moon is between the sun and the earth; under these conditions (the two bodies acting in concert) we have spring tides in the ocean, and a maximum of attraction on the mass of the earth. Hence the crust, which at the time referred to was under tremendous strain, only required the addition of that caused by the lunar and solar attractions to produce rupture in both cases, giving rise to increased activity, and the extrusion of lava and volatile matter. It may, in general, be safely affirmed that low barometric pressure on the one hand, and the occurrence of the syzygies (when the attractions of the sun and moon are in the same line) on the other, have had great influence in determining the crises of eruptions of volcanic mountains when in a state of unrest.
Contrast between the Northern and Southern Slopes.—Before leaving Vesuvius it may be observed that throughout all the eruptions of modern times the northern side of the mountain, that is the old crater and flank of Somma, has been secure from the lava-flows, and has enjoyed an immunity which does not belong to the southern and western side. If we look at a map of the mountain showing the direction of the streams during the last three centuries,[13] we observe that all the streams of that period flowed down on the side overlooking the Bay of Naples; on the opposite side the wall of Monte di Somma presents an unbroken front to the lava-streams. From this it may be inferred that one side, the west, is weaker than the other; and consequently, when the lava and vapours are being forced upwards, under enormous pressure from beneath, the western side gives way under the strain, as in the case of the fissure of 1872, and the lava and vapours find means of escape. From what has happened in the past it is clear that no place on the western side of the mountain is entirely safe from devastation by floods of lava; while the prevalent winds tend to carry the ashes and lapilli, which are hurled into the air, in the same westerly direction.
[1] For an excellent view of this remarkable volcanic group see Judd's Volcanoes, 4th edition, p. 43.
[2] Plutarch, Life of Cassius; ed. Reiske, vol. iii. p. 240.
[3] Strabo gives the following account of the appearance and condition of Vesuvius in his day:—"Supra haec loca situs est Vesuvius mons, agris cinctus optimis; dempto vertice, qui magna sui parte planus, totus sterilis est, adspectu sinereus, cavernasque ostendens fistularum plenas et lapidum colore fuliginoso, utpote ab igni exesorum. Ut conjectarum facere possis, ista loca quondam arsisse et crateras ignis habuisse, deinde materia deficiente restricta fuisse."—Rer. Geog., lib. v.
[4] A tablet over the entrance records this act of pious liberality, and is given by Phillips, loc. cit., p. 12.
[5] The stone pine, Pinus pinea, which Turner knew how to use with so much effect in his Italian landscapes.
[6] Bulwer Lytton's Last Days of Pompeii presents to the reader a graphic picture of the terrible event here referred to:—"The eyes of the crowd followed the gesture of the Egyptian, and beheld with ineffable dismay a vast vapour shooting from the summit of Vesuvius, in the form of a gigantic pine tree; the trunk—blackness, the branches—fire! A fire that shifted and wavered in its hues with every moment—now fiercely luminous, now of a dull and dying red that again blazed terrifically forth with intolerable glare!... Then there arose on high the shrieks of women; the men stared at each other, but were speechless. At that moment they felt the earth shake beneath their feet; the walls of the theatre trembled; and beyond, in the distance, they heard the crash of falling roofs; an instant more and the mountain-cloud seemed to roll towards them, dark and rapid; at the same time it cast forth from its bosom a shower of ashes mixed with vast fragments of burning stone. Over the crushing vines—over the desolate streets—over the amphitheatre itself—far and wide, with many a mighty splash in the agitated sea, fell that awful shower." A visit to the disinterred city will probably produce on the mind a still more lasting and vivid impression of the swift destruction which overtook this city. |
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