As we have seen that crystals of feldspar, in the instance described by Von Buch, sink in obsidian, in accordance with their known greater specific gravity, we might expect to find in every trachytic district, where obsidian has flowed as lava, that it had proceeded from the upper or highest orifices. This, according to Von Buch, holds good in a remarkable manner both at the Lipari Islands and on the Peak of Teneriffe; at this latter place obsidian has never flowed from a less height than 9,200 feet. Obsidian, also, appears to have been erupted from the loftiest peaks of the Peruvian Cordillera. I will only further observe, that the specific gravity of quartz varies from 2.6 to 2.8; and therefore, that when present in a volcanic focus, it would not tend to sink with the basaltic bases; and this, perhaps, explains the frequent presence, and the abundance of this mineral, in the lavas of the trachytic series, as observed in previous parts of this volume.

An objection to the foregoing theory will, perhaps, be drawn from the plutonic rocks not being separated into two evidently distinct series, of different specific gravities; although, like the volcanic, they have been liquified. In answer, it may first be remarked, that we have no evidence of the atoms of any one of the constituent minerals in the plutonic series having been aggregated, whilst the others remained fluid, which we have endeavoured to show is an almost necessary condition of their separation; on the contrary, the crystals have generally impressed each other with their forms. (The crystalline paste of phonolite is frequently penetrated by long needles of hornblende; from which it appears that the hornblende, though the more fusible mineral, has crystallised before, or at the same time with a more refractory substance. Phonolite, as far as my observations serve, in every instance appears to be an injected rock, like those of the plutonic series; hence probably, like these latter, it has generally been cooled without repeated and violent disturbances. Those geologists who have doubted whether granite could have been formed by igneous liquefaction, because minerals of different degrees of fusibility impress each other with their forms, could not have been aware of the fact of crystallised hornblende penetrating phonolite, a rock undoubtedly of igneous origin. The viscidity, which it is now known, that both feldspar and quartz retain at a temperature much below their points of fusion, easily explains their mutual impressment. Consult on this subject Mr. Horner's paper on Bonn "Geolog. Transact." volume 4 page 439; and "L'Institut" with respect to quartz 1839 page 161.)

In the second place, the perfect tranquillity, under which it is probable that the plutonic masses, buried at profound depths, have cooled, would, most likely, be highly unfavourable to the separation of their constituent minerals; for, if the attractive force, which during the progressive cooling draws together the molecules of the different minerals, has power sufficient to keep them together, the friction between such half-formed crystals or pasty globules would effectually prevent the heavier ones from sinking, or the lighter ones from rising. On the other hand, a small amount of disturbance, which would probably occur in most volcanic foci, and which we have seen does not prevent the separation of granules of lead from a mixture of molten lead and silver, or crystals of feldspar from streams of lava, by breaking and dissolving the less perfectly formed globules, would permit the more perfect and therefore unbroken crystals, to sink or rise, according to their specific gravity.

Although in plutonic rocks two distinct species, corresponding to the trachytic and basaltic series, do not exist, I much suspect that a certain amount of separation of their constituent parts has often taken place. I suspect this from having observed how frequently dikes of greenstone and basalt intersect widely extended formations of granite and the allied metamorphic rocks. I have never examined a district in an extensive granitic region without discovering dikes; I may instance the numerous trap-dikes, in several districts of Brazil, Chile, and Australia, and at the Cape of Good Hope: many dikes likewise occur in the great granitic tracts of India, in the north of Europe, and in other countries. Whence, then, has the greenstone and basalt, forming these dikes, come? Are we to suppose, like some of the elder geologists, that a zone of trap is uniformly spread out beneath the granitic series, which composes, as far as we know, the foundations of the earth's crust? Is it not more probable, that these dikes have been formed by fissures penetrating into partially cooled rocks of the granitic and metamorphic series, and by their more fluid parts, consisting chiefly of hornblende, oozing out, and being sucked into such fissures? At Bahia, in Brazil, in a district composed of gneiss and primitive greenstone, I saw many dikes, of a dark augitic (for one crystal certainly was of this mineral) or hornblendic rock, which, as several appearances clearly proved, either had been formed before the surrounding mass had become solid, or had together with it been afterwards thoroughly softened. (Portions of these dikes have been broken off, and are now surrounded by the primary rocks, with their laminae conformably winding round them. Dr. Hubbard also ("Silliman's Journal" volume 34 page 119), has described an interlacement of trap-veins in the granite of the White Mountains, which he thinks must have been formed when both rocks were soft.) On both sides of one of these dikes, the gneiss was penetrated, to the distance of several yards, by numerous, curvilinear threads or streaks of dark matter, which resembled in form clouds of the class called cirrhi- comae; some few of these threads could be traced to their junction with the dike. When examining them, I doubted whether such hair-like and curvilinear veins could have been injected, and I now suspect, that instead of having been injected from the dike, they were its feeders. If the foregoing views of the origin of trap-dikes in widely extended granitic regions far from rocks of any other formation, be admitted as probable, we may further admit, in the case of a great body of plutonic rock, being impelled by repeated movements into the axis of a mountain-chain, that its more liquid constituent parts might drain into deep and unseen abysses; afterwards, perhaps, to be brought to the surface under the form, either of injected masses of greenstone and augitic porphyry, or of basaltic eruptions. (Mr. Phillips "Lardner's Encyclop." volume 2 page 115 quotes Von Buch's statement, that augitic porphyry ranges parallel to, and is found constantly at the base of, great chains of mountains. Humboldt, also, has remarked the frequent occurrence of trap-rock, in a similar position; of which fact I have observed many examples at the foot of the Chilian Cordillera. The existence of granite in the axes of great mountain chains is always probable, and I am tempted to suppose, that the laterally injected masses of augitic porphyry and of trap, bear nearly the same relation to the granitic axes which basaltic lavas bear to the central trachytic masses, round the flanks of which they have so frequently been erupted.) Much of the difficulty which geologists have experienced when they have compared the composition of volcanic with plutonic formations, will, I think, be removed, if we may believe that most plutonic masses have been, to a certain extent, drained of those comparatively weighty and easily liquified elements, which compose the trappean and basaltic series of rocks.

ON THE DISTRIBUTION OF VOLCANIC ISLANDS.

During my investigations on coral-reefs, I had occasion to consult the works of many voyagers, and I was invariably struck with the fact, that with rare exceptions, the innumerable islands scattered throughout the Pacific, Indian, and Atlantic Oceans, were composed either of volcanic, or of modern coral-rocks. It would be tedious to give a long catalogue of all the volcanic islands; but the exceptions which I have found are easily enumerated: in the Atlantic, we have St. Paul's Rock, described in this volume, and the Falkland Islands, composed of quartz and clay-slate; but these latter islands are of considerable size, and lie not very far from the South American coast (Judging from Forster's imperfect observation, perhaps Georgia is not volcanic. Dr. Allan is my informant with regard to the Seychelles. I do not know of what formation Rodriguez, in the Indian Ocean, is composed.): in the Indian Ocean, the Seychelles (situated in a line prolonged from Madagascar) consist of granite and quartz: in the Pacific Ocean, New Caledonia, an island of large size, belongs (as far as is known) to the primary class. New Zealand, which contains much volcanic rock and some active volcanoes, from its size cannot be classed with the small islands, which we are now considering. The presence of a small quantity of non-volcanic rock, as of clay-slate on three of the Azores (This is stated on the authority of Count V. de Bedemar, with respect to Flores and Graciosa (Charlsworth "Magazine of Nat. Hist." volume 1 page 557). St. Maria has no volcanic rock, according to Captain Boyd (Von Buch "Descript." page 365). Chatham Island has been described by Dr. Dieffenbach in the "Geographical Journal" 1841 page 201. As yet we have received only imperfect notices on Kerguelen Land, from the Antarctic Expedition.), or of tertiary limestone at Madeira, or of clay-slate at Chatham Island in the Pacific, or of lignite at Kerguelen Land, ought not to exclude such islands or archipelagoes, if formed chiefly of erupted matter, from the volcanic class.

The composition of the numerous islands scattered through the great oceans being with such rare exceptions volcanic, is evidently an extension of that law, and the effect of those same causes, whether chemical or mechanical, from which it results, that a vast majority of the volcanoes now in action stand either as islands in the sea, or near its shores. This fact of the ocean-islands being so generally volcanic is also interesting in relation to the nature of the mountain-chains on our continents, which are comparatively seldom volcanic; and yet we are led to suppose that where our continents now stand an ocean once extended. Do volcanic eruptions, we may ask, reach the surface more readily through fissures formed during the first stages of the conversion of the bed of the ocean into a tract of land?

Looking at the charts of the numerous volcanic archipelagoes, we see that the islands are generally arranged either in single, double, or triple rows, in lines which are frequently curved in a slight degree. (Professors William and Henry Darwin Rogers have lately insisted much, in a memoir read before the American Association, on the regularly curved lines of elevation in parts of the Appalachian range.) Each separate island is either rounded, or more generally elongated in the same direction with the group in which it stands, but sometimes transversely to it. Some of the groups which are not much elongated present little symmetry in their forms; M. Virlet ("Bulletin de la Soc. Geolog." tome 3 page 110.) states that this is the case with the Grecian Archipelago: in such groups I suspect (for I am aware how easy it is to deceive oneself on these points), that the vents are generally arranged on one line, or on a set of short parallel lines, intersecting at nearly right angles another line, or set of lines. The Galapagos Archipelago offers an example of this structure, for most of the islands and the chief orifices on the largest island are so grouped as to fall on a set of lines ranging about N.W. by N., and on another set ranging about W.S.W.: in the Canary Archipelago we have a simpler structure of the same kind: in the Cape de Verde group, which appears to be the least symmetrical of any oceanic volcanic archipelago, a N.W. and S.E. line formed by several islands, if prolonged, would intersect at right angles a curved line, on which the remaining islands are placed.

Von Buch ("Description des Isles Canaries" page 324.) has classed all volcanoes under two heads, namely, CENTRAL VOLCANOES, round which numerous eruptions have taken place on all sides, in a manner almost regular, and VOLCANIC CHAINS. In the examples given of the first class, as far as position is concerned, I can see no grounds for their being called "central;" and the evidence of any difference in mineralogical nature between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No doubt some one island in most small volcanic archipelagoes is apt to be considerably higher than the others; and in a similar manner, whatever the cause may be, that on the same island one vent is generally higher than all the others. Von Buch does not include in his class of volcanic chains small archipelagoes, in which the islands are admitted by him, as at the Azores, to be arranged in lines; but when viewing on a map of the world how perfect a series exists from a few volcanic islands placed in a row to a train of linear archipelagoes following each other in a straight line, and so on to a great wall like the Cordillera of America, it is difficult to believe that there exists any essential difference between short and long volcanic chains. Von Buch (Idem page 393.) states that his volcanic chains surmount, or are closely connected with, mountain-ranges of primary formation: but if trains of linear archipelagoes are, in the course of time, by the long- continued action of the elevatory and volcanic forces, converted into mountain-ranges, it would naturally result that the inferior primary rocks would often be uplifted and brought into view.

Some authors have remarked that volcanic islands occur scattered, though at very unequal distances, along the shores of the great continents, as if in some measure connected with them. In the case of Juan Fernandez, situated 330 miles from the coast of Chile, there was undoubtedly a connection between the volcanic forces acting under this island and under the continent, as was shown during the earthquake of 1835. The islands, moreover, of some of the small volcanic groups which thus border continents, are placed in lines, related to those along which the adjoining shores of the continents trend; I may instance the lines of intersection at the Galapagos, and at the Cape de Verde Archipelagoes, and the best marked line of the Canary Islands. If these facts be not merely accidental, we see that many scattered volcanic islands and small groups are related not only by proximity, but in the direction of the fissures of eruption to the neighbouring continents—a relation, which Von Buch considers, characteristic of his great volcanic chains.

In volcanic archipelagoes, the orifices are seldom in activity on more than one island at a time; and the greater eruptions usually recur only after long intervals. Observing the number of craters, that are usually found on each island of a group, and the vast amount of matter which has been erupted from them, one is led to attribute a high antiquity even to those groups, which appear, like the Galapagos, to be of comparatively recent origin. This conclusion accords with the prodigious amount of degradation, by the slow action of the sea, which their originally sloping coasts must have suffered, when they are worn back, as is so often the case, into grand precipices. We ought not, however, to suppose, in hardly any instance, that the whole body of matter, forming a volcanic island, has been erupted at the level, on which it now stands: the number of dikes, which seem invariably to intersect the interior parts of every volcano, show, on the principles explained by M. Elie de Beaumont, that the whole mass has been uplifted and fissured. A connection, moreover, between volcanic eruptions and contemporaneous elevations in mass has, I think, been shown to exist in my work on Coral-Reefs, both from the frequent presence of upraised organic remains, and from the structure of the accompanying coral-reefs. (A similar conclusion is forced on us, by the phenomena, which accompanied the earthquake of 1835, at Concepcion, and which are detailed in my paper (volume 5 page 601) in the "Geological Transactions.") Finally, I may remark, that in the same Archipelago, eruptions have taken place within the historical period on more than one of the parallel lines of fissure: thus, at the Galapagos Archipelago, eruptions have taken place from a vent on Narborough Island, and from one on Albemarle Island, which vents do not fall on the same line; at the Canary Islands, eruptions have taken place in Teneriffe and Lanzarote; and at the Azores, on the three parallel lines of Pico, St. Jorge, and Terceira. Believing that a mountain-axis differs essentially from a volcano, only in plutonic rocks having been injected, instead of volcanic matter having been ejected, this appears to me an interesting circumstance; for we may infer from it as probable, that in the elevation of a mountain-chain, two or more of the parallel lines forming it may be upraised and injected within the same geological period.

CHAPTER VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.

New South Wales. Sandstone formation. Embedded pseudo-fragments of shale. Stratification. Current-cleavage. Great valleys. Van Diemen's Land. Palaeozoic formation. Newer formation with volcanic rocks. Travertin with leaves of extinct plants. Elevation of the land. New Zealand. King George's Sound. Superficial ferruginous beds. Superficial calcareous deposits, with casts of branches. Their origin from drifted particles of shells and corals. Their extent. Cape of Good Hope. Junction of the granite and clay-slate. Sandstone formation.

The "Beagle," in her homeward voyage, touched at New Zealand, Australia, Van Diemen's Land, and the Cape of Good Hope. In order to confine the Third Part of these Geological Observations to South America, I will here briefly describe all that I observed at these places worthy of the attention of geologists.

NEW SOUTH WALES.

My opportunities of observation consisted of a ride of ninety geographical miles to Bathurst, in a W.N.W. direction from Sydney. The first thirty miles from the coast passes over a sandstone country, broken up in many places by trap-rocks, and separated by a bold escarpment overhanging the river Nepean, from the great sandstone platform of the Blue Mountains. This upper platform is 1,000 feet high at the edge of the escarpment, and rises in a distance of twenty-five miles to between three and four thousand feet above the level of the sea. At this distance the road descends to a country rather less elevated, and composed in chief part of primary rocks. There is much granite, in one part passing into a red porphyry with octagonal crystals of quartz, and intersected in some places by trap-dikes. Near the Downs of Bathurst I passed over much pale-brown, glossy clay-slate, with the shattered laminae running north and south; I mention this fact, because Captain King informs me that, in the country a hundred miles southward, near Lake George, the mica-slate ranges so invariably north and south that the inhabitants take advantage of it in finding their way through the forests.

The sandstone of the Blue Mountains is at least 1,200 feet thick, and in some parts is apparently of greater thickness; it consists of small grains of quartz, cemented by white earthy matter, and it abounds with ferruginous veins. The lower beds sometimes alternate with shales and coal: at Wolgan I found in carbonaceous shale leaves of the Glossopteris Brownii, a fern which so frequently accompanies the coal of Australia. The sandstone contains pebbles of quartz; and these generally increase in number and size (seldom, however, exceeding an inch or two in diameter) in the upper beds: I observed a similar circumstance in the grand sandstone formation at the Cape of Good Hope. On the South American coast, where tertiary and supra- tertiary beds have been extensively elevated, I repeatedly noticed that the uppermost beds were formed of coarser materials than the lower: this appears to indicate that, as the sea became shallower, the force of the waves or currents increased. On the lower platform, however, between the Blue Mountains and the coast, I observed that the upper beds of the sandstone frequently passed into argillaceous shale,—the effect, probably, of this lower space having been protected from strong currents during its elevation. The sandstone of the Blue Mountains evidently having been of mechanical origin, and not having suffered any metamorphic action, I was surprised at observing that, in some specimens, nearly all the grains of quartz were so perfectly crystallised with brilliant facets that they evidently had not in their PRESENT form been aggregated in any previously existing rock. (I have lately seen, in a paper by Smith (the father of English geologists), in the "Magazine of Natural History," that the grains of quartz in the millstone grit of England are often crystallised. Sir David Brewster, in a paper read before the British Association, 1840, states, that in old decomposed glass, the silex and metals separate into concentric rings, and that the silex regains its crystalline structure, as is shown by its action on light.) It is difficult to imagine how these crystals could have been formed; one can hardly believe that they were separately precipitated in their present crystallised state. Is it possible that rounded grains of quartz may have been acted on by a fluid corroding their surfaces, and depositing on them fresh silica? I may remark that, in the sandstone formation of the Cape of Good Hope, it is evident that silica has been profusely deposited from aqueous solution.

In several parts of the sandstone I noticed patches of shale which might at the first glance have been mistaken for extraneous fragments; their horizontal laminae, however, being parallel with those of the sandstone, showed that they were the remnants of thin, continuous beds. One such fragment (probably the section of a long narrow strip) seen in the face of a cliff, was of greater vertical thickness than breadth, which proves that this bed of shale must have been in some slight degree consolidated, after having been deposited, and before being worn away by the currents. Each patch of the shale shows, also, how slowly many of the successive layers of sandstone were deposited. These pseudo-fragments of shale will perhaps explain, in some cases, the origin of apparently extraneous fragments in crystalline metamorphic rocks. I mention this, because I found near Rio de Janeiro a well-defined angular fragment, seven yards long by two yards in breadth, of gneiss containing garnets and mica in layers, enclosed in the ordinary, stratified, porphyritic gneiss of the country. The laminae of the fragment and of the surrounding matrix ran in exactly the same direction, but they dipped at different angles. I do not wish to affirm that this singular fragment (a solitary case, as far as I know) was originally deposited in a layer, like the shale in the Blue Mountains, between the strata of the porphyritic gneiss, before they were metamorphosed; but there is sufficient analogy between the two cases to render such an explanation possible.

STRATIFICATION OF THE ESCARPMENT.

The strata of the Blue Mountains appear to the eye horizontal; but they probably have a similar inclination with the surface of the platform, which slopes from the west towards the escarpment over the Nepean, at an angle of one degree, or of one hundred feet in a mile. (This is stated on the authority of Sir T. Mitchell in "Travels" volume 2 page 357.) The strata of the escarpment dip almost conformably with its steeply inclined face, and with so much regularity, that they appear as if thrown into their present position; but on a more careful examination, they are seen to thicken and to thin out, and in the upper part to be succeeded and almost capped by horizontal beds. These appearances render it probable, that we here see an original escarpment, not formed by the sea having eaten back into the strata, but by the strata having originally extended only thus far. Those who have been in the habit of examining accurate charts of sea-coasts, where sediment is accumulating, will be aware, that the surfaces of the banks thus formed, generally slope from the coast very gently towards a certain line in the offing, beyond which the depth in most cases suddenly becomes great. I may instance the great banks of sediment within the West Indian Archipelago (I have described these very curious banks in the Appendix to my volume on the structure of Coral-Reefs. I have ascertained the inclination of the edges of the banks, from information given me by Captain B. Allen, one of the surveyors, and by carefully measuring the horizontal distances between the last sounding on the bank and the first in the deep water. Widely extended banks in all parts of the West Indies have the same general form of surface.), which terminate in submarine slopes, inclined at angles of between thirty and forty degrees, and sometimes even at more than forty degrees: every one knows how steep such a slope would appear on the land. Banks of this nature, if uplifted, would probably have nearly the same external form as the platform of the Blue Mountains, where it abruptly terminates over the Nepean.

CURRENT-CLEAVAGE.

The strata of sandstone in the low coast country, and likewise on the Blue Mountains, are often divided by cross or current laminae, which dip in different directions, and frequently at an angle of forty-five degrees. Most authors have attributed these cross layers to successive small accumulations on an inclined surface; but from a careful examination in some parts of the New Red Sandstone of England, I believe that such layers generally form parts of a series of curves, like gigantic tidal ripples, the tops of which have since been cut off, either by nearly horizontal layers, or by another set of great ripples, the folds of which do not exactly coincide with those below them. It is well-known to surveyors that mud and sand are disturbed during storms at considerable depths, at least from three hundred to four hundred and fifty feet (See Martin White on "Soundings in the British Channel" pages 4 and 166.), so that the nature of the bottom even becomes temporarily changed; the bottom, also, at a depth between sixty and seventy feet, has been observed to be broadly rippled. (M. Siau on the "Action of Waves" "Edin. New Phil. Journ." volume 31 page 245.) One may, therefore, be allowed to suspect, from the appearance just mentioned in the New Red Sandstone, that at greater depths, the bed of the ocean is heaped up during gales into great ripple-like furrows and depressions, which are afterwards cut off by the currents during more tranquil weather, and again furrowed during gales.

VALLEYS IN THE SANDSTONE PLATFORMS.

The grand valleys, by which the Blue Mountains and the other sandstone platforms of this part of Australia are penetrated, and which long offered an insuperable obstacle to the attempts of the most enterprising colonist to reach the interior country, form the most striking feature in the geology of New South Wales. They are of grand dimensions, and are bordered by continuous links of lofty cliffs. It is not easy to conceive a more magnificent spectacle, than is presented to a person walking on the summit- plains, when without any notice he arrives at the brink of one of these cliffs, which are so perpendicular, that he can strike with a stone (as I have tried) the trees growing, at the depth of between one thousand and one thousand five hundred feet below him; on both hands he sees headland beyond headland of the receding line of cliff, and on the opposite side of the valley, often at the distance of several miles, he beholds another line rising up to the same height with that on which he stands, and formed of the same horizontal strata of pale sandstone. The bottoms of these valleys are moderately level, and the fall of the rivers flowing in them, according to Sir T. Mitchell, is gentle. The main valleys often send into the platform great baylike arms, which expand at their upper ends; and on the other hand, the platform often sends promontories into the valley, and even leaves in them great, almost insulated, masses. So continuous are the bounding lines of cliff, that to descend into some of these valleys, it is necessary to go round twenty miles; and into others, the surveyors have only lately penetrated, and the colonists have not yet been able to drive in their cattle. But the most remarkable point of structure in these valleys, is, that although several miles wide in their upper parts, they generally contract towards their mouths to such a degree as to become impassable. The Surveyor-General, Sir T. Mitchell, in vain endeavoured, first on foot and then by crawling between the great fallen fragments of sandstone, to ascend through the gorge by which the river Grose joins the Nepean ("Travels in Australia" volume 1 page 154.—I must express my obligation to Sir T. Mitchell for several interesting personal communications on the subject of these great valleys of New South Wales.); yet the valley of the Grose in its upper part, as I saw, forms a magnificent basin some miles in width, and is on all sides surrounded by cliffs, the summits of which are believed to be nowhere less than 3,000 feet above the level of the sea. When cattle are driven into the valley of the Wolgan by a path (which I descended) partly cut by the colonists, they cannot escape; for this valley is in every other part surrounded by perpendicular cliffs, and eight miles lower down, it contracts, from an average width of half a mile, to a mere chasm impassable to man or beast. Sir T. Mitchell states, that the great valley of the Cox river with all its branches contracts, where it unites with the Nepean, into a gorge 2,200 yards wide, and about one thousand feet in depth. (Idem volume 2 page 358.) Other similar cases might have been added.

The first impression, from seeing the correspondence of the horizontal strata, on each side of these valleys and great amphitheatre-like depressions, is that they have been in chief part hollowed out, like other valleys, by aqueous erosion; but when one reflects on the enormous amount of stone, which on this view must have been removed, in most of the above cases through mere gorges or chasms, one is led to ask whether these spaces may not have subsided. But considering the form of the irregularly branching valleys, and of the narrow promontories, projecting into them from the platforms, we are compelled to abandon this notion. To attribute these hollows to alluvial action, would be preposterous; nor does the drainage from the summit-level always fall, as I remarked near the Weatherboard, into the head of these valleys, but into one side of their bay-like recesses. Some of the inhabitants remarked to me, that they never viewed one of these baylike recesses, with the headlands receding on both hands, without being struck with their resemblance to a bold sea-coast. This is certainly the case; moreover, the numerous fine harbours, with their widely branching arms, on the present coast of New South Wales, which are generally connected with the sea by a narrow mouth, from one mile to a quarter of a mile in width, passing through the sandstone coast-cliffs, present a likeness, though on a miniature scale, to the great valleys of the interior. But then immediately occurs the startling difficulty, why has the sea worn out these great, though circumscribed, depressions on a wide platform, and left mere gorges, through which the whole vast amount of triturated matter must have been carried away? The only light I can throw on this enigma, is by showing that banks appear to be forming in some seas of the most irregular forms, and that the sides of such banks are so steep (as before stated) that a comparatively small amount of subsequent erosion would form them into cliffs: that the waves have power to form high and precipitous cliffs, even in landlocked harbours, I have observed in many parts of South America. In the Red Sea, banks with an extremely irregular outline and composed of sediment, are penetrated by the most singularly shaped creeks with narrow mouths: this is likewise the case, though on a larger scale, with the Bahama Banks. Such banks, I have been led to suppose, have been formed by currents heaping sediment on an irregular bottom. (See the "Appendix" to the Part on Coral-Reefs. The fact of the sea heaping up mud round a submarine nucleus, is worthy of the notice of geologists: for outlyers of the same composition with the coast banks are thus formed; and these, if upheaved and worn into cliffs, would naturally be thought to have been once connected together.) That in some cases, the sea, instead of spreading out sediment in a uniform sheet, heaps it round submarine rocks and islands, it is hardly possible to doubt, after having examined the charts of the West Indies. To apply these ideas to the sandstone platforms of New South Wales, I imagine that the strata might have been heaped on an irregular bottom by the action of strong currents, and of the undulations of an open sea; and that the valley-like spaces thus left unfilled might, during a slow elevation of the land, have had their steeply sloping flanks worn into cliffs; the worn-down sandstone being removed, either at the time when the narrow gorges were cut by the retreating sea, or subsequently by alluvial action.

VAN DIEMEN'S LAND.

The southern part of this island is mainly formed of mountains of greenstone, which often assumes a syenitic character, and contains much hypersthene. These mountains, in their lower half, are generally encased by strata containing numerous small corals and some shells. These shells have been examined by Mr. G.B. Sowerby, and have been described by him: they consist of two species of Producta, and of six of Spirifera; two of these, namely, P. rugata and S. rotundata, resemble, as far as their imperfect condition allows of comparison, British mountain-limestone shells. Mr. Lonsdale has had the kindness to examine the corals; they consist of six undescribed species, belonging to three genera. Species of these genera occur in the Silurian, Devonian, and Carboniferous strata of Europe. Mr. Lonsdale remarks, that all these fossils have undoubtedly a Palaeozoic character, and that probably they correspond in age to a division of the system above the Silurian formations.

The strata containing these remains are singular from the extreme variability of their mineralogical composition. Every intermediate form is present, between flinty-slate, clay-slate passing into grey wacke, pure limestone, sandstone, and porcellanic rock; and some of the beds can only be described as composed of a siliceo-calcareo-clay-slate. The formation, as far as I could judge, is at least a thousand feet in thickness: the upper few hundred feet usually consist of a siliceous sandstone, containing pebbles and no organic remains; the inferior strata, of which a pale flinty slate is perhaps the most abundant, are the most variable; and these chiefly abound with the remains. Between two beds of hard crystalline limestone, near Newtown, a layer of white soft calcareous matter is quarried, and is used for whitewashing houses. From information given to me by Mr. Frankland, the Surveyor-General, it appears that this Palaeozoic formation is found in different parts of the whole island; from the same authority, I may add, that on the north-eastern coast and in Bass' Straits primary rocks extensively occur.

The shores of Storm Bay are skirted, to the height of a few hundred feet, by strata of sandstone, containing pebbles of the formation just described, with its characteristic fossils, and therefore belonging to a subsequent age. These strata of sandstone often pass into shale, and alternate with layers of impure coal; they have in many places been violently disturbed. Near Hobart Town, I observed one dike, nearly a hundred yards in width, on one side of which the strata were tilted at an angle of 60 degrees, and on the other they were in some parts vertical, and had been altered by the effects of the heat. On the west side of Storm Bay, I found these strata capped by streams of basaltic lava with olivine; and close by there was a mass of brecciated scoriae, containing pebbles of lava, which probably marks the place of an ancient submarine crater. Two of these streams of basalt were separated from each other by a layer of argillaceous wacke, which could be traced passing into partially altered scoriae. The wacke contained numerous rounded grains of a soft, grass-green mineral, with a waxy lustre, and translucent on its edges: under the blowpipe it instantly blackened, and the points fused into a strongly magnetic, black enamel. In these characters, it resembles those masses of decomposed olivine, described at St. Jago in the Cape de Verde group; and I should have thought that it had thus originated, had I not found a similar substance, in cylindrical threads, within the cells of the vesicular basalt,—a state under which olivine never appears; this substance, I believe, would be classed as bole by mineralogists. (Chlorophaeite, described by Dr. MacCulloch ("Western Islands" volume 1 page 504) as occurring in a basaltic amygdaloid, differs from this substance, in remaining unchanged before the blowpipe, and in blackening from exposure to the air. May we suppose that olivine, in undergoing the remarkable change described at St. Jago, passes through several states?)

TRAVERTIN WITH EXTINCT PLANTS.

Behind Hobart Town there is a small quarry of a hard travertin, the lower strata of which abound with distinct impressions of leaves. Mr. Robert Brown has had the kindness to look at my specimens, and he informed me that there are four or five kinds, none of which he recognises as belonging to existing species. The most remarkable leaf is palmate, like that of a fan- palm, and no plant having leaves of this structure has hitherto been discovered in Van Diemen's Land. The other leaves do not resemble the most usual form of the Eucalyptus (of which tribe the existing forests are chiefly composed), nor do they resemble that class of exceptions to the common form of the leaves of the Eucalyptus, which occur in this island. The travertin containing this remnant of a lost vegetation, is of a pale yellow colour, hard, and in parts even crystalline; but not compact, and is everywhere penetrated by minute, tortuous, cylindrical pores. It contains a very few pebbles of quartz, and occasionally layers of chalcedonic nodules, like those of chert in our Greensand. From the pureness of this calcareous rock, it has been searched for in other places, but has never been found. From this circumstance, and from the character of the deposit, it was probably formed by a calcareous spring entering a small pool or narrow creek. The strata have subsequently been tilted and fissured; and the surface has been covered by a singular mass, with which, also, a large fissure has been filled up, formed of balls of trap embedded in a mixture of wacke and a white, earthy, alumino-calcareous substance. Hence it would appear, as if a volcanic eruption had taken place on the borders of the pool, in which the calcareous matter was depositing, and had broken it up and drained it.

ELEVATION OF THE LAND.

Both the eastern and western shores of the bay, in the neighbourhood of Hobart Town, are in most parts covered to the height of thirty feet above the level of high-water mark, with broken shells, mingled with pebbles. The colonists attribute these shells to the aborigines having carried them up for food: undoubtedly, there are many large mounds, as was pointed out to me by Mr. Frankland, which have been thus formed; but I think from the numbers of the shells, from their frequent small size, from the manner in which they are thinly scattered, and from some appearances in the form of the land, that we must attribute the presence of the greater number to a small elevation of the land. On the shore of Ralph Bay (opening into Storm Bay) I observed a continuous beach about fifteen feet above high-water mark, clothed with vegetation, and by digging into it, pebbles encrusted with Serpulae were found: along the banks, also, of the river Derwent, I found a bed of broken sea-shells above the surface of the river, and at a point where the water is now much too fresh for sea-shells to live; but in both these cases, it is just possible, that before certain spits of sand and banks of mud in Storm Bay were accumulated, the tides might have risen to the height where we now find the shells. ( It would appear that some changes are now in progress in Ralph Bay, for I was assured by an intelligent farmer, that oysters were formerly abundant in it, but that about the year 1834 they had, without any apparent cause, disappeared. In the "Transactions of the Maryland Academy" volume 1 part 1 page 28 there is an account by Mr. Ducatel of vast beds of oysters and clams having been destroyed by the gradual filling up of the shallow lagoons and channels, on the shores of the southern United States. At Chiloe, in South America, I heard of a similar loss, sustained by the inhabitants, in the disappearance from one part of the coast of an edible species of Ascidia.)

Evidence more or less distinct of a change of level between the land and water, has been detected on almost all the land on this side of the globe. Captain Grey, and other travellers, have found in Southern Australia upraised shells, belonging either to the recent, or to a late tertiary period. The French naturalists in Baudin's expedition, found shells similarly circumstanced on the S.W. coast of Australia. The Rev. W.B. Clarke finds proofs of the elevation of the land, to the amount of 400 feet, at the Cape of Good Hope. ("Proceedings of the Geological Society" volume 3 page 420.) In the neighbourhood of the Bay of Islands in New Zealand, I observed that the shores were scattered to some height, as at Van Diemen's Land, with sea-shells, which the colonists attribute to the natives. (I will here give a catalogue of the rocks which I met with near the Bay of Islands, in New Zealand:—1st, Much basaltic lava, and scoriform rocks, forming distinct craters;—2nd, A castellated hill of horizontal strata of flesh-coloured limestone, showing when fractured distinct crystalline facets: the rain has acted on this rock in a remarkable manner, corroding its surface into a miniature model of an Alpine country: I observed here layers of chert and clay ironstone; and in the bed of a stream, pebbles of clay-slate;—3rd, The shores of the Bay of Islands are formed of a feldspathic rock, of a bluish-grey colour, often much decomposed, with an angular fracture, and crossed by numerous ferruginous seams, but without any distinct stratification or cleavage. Some varieties are highly crystalline, and would at once be pronounced to be trap; others strikingly resembled clay-slate, slightly altered by heat: I was unable to form any decided opinion on this formation.) Whatever may have been the origin of these shells, I cannot doubt, after having seen a section of the valley of the Thames River (37 degrees S.), drawn by the Rev. W. Williams, that the land has been there elevated: on the opposite sides of this great valley, three step-like terraces, composed of an enormous accumulation of rounded pebbles, exactly correspond with each other: the escarpment of each terrace is about fifty feet in height. No one after having examined the terraces in the valleys on the western shores of South America, which are strewed with sea-shells, and have been formed during intervals of rest in the slow elevation of the land, could doubt that the New Zealand terraces have been similarly formed. I may add, that Dr. Dieffenbach, in his description of the Chatham Islands ("Geographical Journal" volume 11 pages 202, 205.) (S.W. of New Zealand), states that it is manifest "that the sea has left many places bare which were once covered by its waters."

KING GEORGE'S SOUND.

This settlement is situated at the south-western angle of the Australian continent: the whole country is granitic, with the constituent minerals sometimes obscurely arranged in straight or curved laminae. In these cases, the rock would be called by Humboldt, gneiss-granite, and it is remarkable that the form of the bare conical hills, appearing to be composed of great folding layers, strikingly resembles, on a small scale, those composed of gneiss-granite at Rio de Janeiro, and those described by Humboldt at Venezuela. These plutonic rocks are, in many places, intersected by trappean-dikes; in one place, I found ten parallel dikes ranging in an E. and W. line; and not far off another set of eight dikes, composed of a different variety of trap, ranging at right angles to the former ones. I have observed in several primary districts, the occurrence of systems of dikes parallel and close to each other.

SUPERFICIAL FERRUGINOUS BEDS.

The lower parts of the country are everywhere covered by a bed, following the inequalities of the surface, of a honeycombed sandstone, abounding with oxides of iron. Beds of nearly similar composition are common, I believe, along the whole western coast of Australia, and on many of the East Indian islands. At the Cape of Good Hope, at the base of the mountains formed of granite and capped with sandstone, the ground is everywhere coated either by a fine-grained, rubbly, ochraceous mass, like that at King George's Sound, or by a coarser sandstone with fragments of quartz, and rendered hard and heavy by an abundance of the hydrate of iron, which presents, when freshly broken, a metallic lustre. Both these varieties have a very irregular texture, including spaces either rounded or angular, full of loose sand: from this cause the surface is always honeycombed. The oxide of iron is most abundant on the edges of the cavities, where alone it affords a metallic fracture. In these formations, as well as in many true sedimentary deposits, it is evident that iron tends to become aggregated, either in the form of a shell, or of a network. The origin of these superficial beds, though sufficiently obscure, seems to be due to alluvial action on detritus abounding with iron.

SUPERFICIAL CALCAREOUS DEPOSIT.

A calcareous deposit on the summit of Bald Head, containing branched bodies, supposed by some authors to have been corals, has been celebrated by the descriptions of many distinguished voyagers. (I visited this hill, in company with Captain Fitzroy, and we came to a similar conclusion regarding these branching bodies.) It folds round and conceals irregular hummocks of granite, at the height of 600 feet above the level of the sea. It varies much in thickness; where stratified, the beds are often inclined at high angles, even as much as at thirty degrees, and they dip in all directions. These beds are sometimes crossed by oblique and even-sided laminae. The deposit consists either of a fine, white calcareous powder, in which not a trace of structure can be discovered, or of exceedingly minute, rounded grains, of brown, yellowish, and purplish colours; both varieties being generally, but not always, mixed with small particles of quartz, and being cemented into a more or less perfect stone. The rounded calcareous grains, when heated in a slight degree, instantly lose their colours; in this and in every other respect, closely resembling those minute, equal- sized particles of shells and corals, which at St. Helena have been drifted up the side of the mountains, and have thus been winnowed of all coarser fragments. I cannot doubt that the coloured calcareous particles here have had a similar origin. The impalpable powder has probably been derived from the decay of the rounded particles; this certainly is possible, for on the coast of Peru, I have traced LARGE UNBROKEN shells gradually falling into a substance as fine as powdered chalk. Both of the above-mentioned varieties of calcareous sandstone frequently alternate with, and blend into, thin layers of a hard substalagmitic rock, which, even when the stone on each side contains particles of quartz, is entirely free from them (I adopt this term from Lieutenant Nelson's excellent paper on the Bermuda Islands "Geolog. Trans." volume 5 page 106, for the hard, compact, cream- or brown- coloured stone, without any crystalline structure, which so often accompanies superficial calcareous accumulations. I have observed such superficial beds, coated with substalagmitic rock, at the Cape of Good Hope, in several parts of Chile, and over wide spaces in La Plata and Patagonia. Some of these beds have been formed from decayed shells, but the origin of the greater number is sufficiently obscure. The causes which determine water to dissolve lime, and then soon to redeposit it, are not, I think, known. The surface of the substalagmitic layers appears always to be corroded by the rain-water. As all the above-mentioned countries have a long dry season, compared with the rainy one, I should have thought that the presence of the substalagmitic was connected with the climate, had not Lieutenant Nelson found this substance forming under sea-water. Disintegrated shell seems to be extremely soluble; of which I found good evidence, in a curious rock at Coquimbo in Chile, which consisted of small, pellucid, empty husks, cemented together. A series of specimens clearly showed that these husks had originally contained small rounded particles of shells, which had been enveloped and cemented together by calcareous matter (as often happens on sea-beaches), and which subsequently had decayed, and been dissolved by water, that must have penetrated through the calcareous husks, without corroding them,—of which processes every stage could be seen.): hence we must suppose that these layers, as well as certain vein- like masses, have been formed by rain dissolving the calcareous matter and re-precipitating it, as has happened at St. Helena. Each layer probably marks a fresh surface, when the, now firmly cemented, particles existed as loose sand. These layers are sometimes brecciated and re-cemented, as if they had been broken by the slipping of the sand when soft. I did not find a single fragment of a sea-shell; but bleached shells of the Helix melo, an existing land species, abound in all the strata; and I likewise found another Helix, and the case of an Oniscus.

The branches are absolutely undistinguishable in shape from the broken and upright stumps of a thicket; their roots are often uncovered, and are seen to diverge on all sides; here and there a branch lies prostrate. The branches generally consist of the sandstone, rather firmer than the surrounding matter, with the central parts filled, either with friable, calcareous matter, or with a substalagmitic variety; this central part is also frequently penetrated by linear crevices, sometimes, though rarely, containing a trace of woody matter. These calcareous, branching bodies, appear to have been formed by fine calcareous matter being washed into the casts or cavities, left by the decay of branches and roots of thickets, buried under drifted sand. The whole surface of the hill is now undergoing disintegration, and hence the casts, which are compact and hard, are left projecting. In calcareous sand at the Cape of Good Hope, I found the casts, described by Abel, quite similar to these at Bald Head; but their centres are often filled with black carbonaceous matter not yet removed. It is not surprising, that the woody matter should have been almost entirely removed from the casts on Bald Head; for it is certain, that many centuries must have elapsed since the thickets were buried; at present, owing to the form and height of the narrow promontory, no sand is drifted up, and the whole surface, as I have remarked, is wearing away. We must, therefore, look back to a period when the land stood lower, of which the French naturalists (See M. Peron "Voyage" tome 1 page 204.) found evidence in upraised shells of recent species, for the drifting on Bald Head of the calcareous and quartzose sand, and the consequent embedment of the vegetable remains. There was only one appearance which at first made me doubt concerning the origin of the cast,—namely, that the finer roots from different stems sometimes became united together into upright plates or veins; but when the manner is borne in mind in which fine roots often fill up cracks in hard earth, and that these roots would decay and leave hollows, as well as the stems, there is no real difficulty in this case. Besides the calcareous branches from the Cape of Good Hope, I have seen casts, of exactly the same forms, from Madeira* and from Bermuda; at this latter place, the surrounding calcareous rocks, judging from the specimens collected by Lieutenant Nelson, are likewise similar, as is their subaerial formation. Reflecting on the stratification of the deposit on Bald Head,—on the irregularly alternating layers of substalagmitic rock,—on the uniformly sized, and rounded particles, apparently of sea-shells and corals,—on the abundance of land-shells throughout the mass,—and finally, on the absolute resemblance of the calcareous casts, to the stumps, roots, and branches of that kind of vegetation, which would grow on sand-hillocks, I think there can be no reasonable doubt, notwithstanding the different opinion of some authors, that a true view of their origin has been here given.

*(Dr. J. Macaulay has fully described ("Edinb. New Phil. Journ." volume 29 page 350) the casts from Madeira. He considers (differently from Mr. Smith of Jordan Hill) these bodies to be corals, and the calcareous deposit to be of subaqueous origin. His arguments chiefly rest (for his remarks on their structure are vague) on the great quantity of the calcareous matter, and on the casts containing animal matter, as shown by their evolving ammonia. Had Dr. Macaulay seen the enormous masses of rolled particles of shells and corals on the beach of Ascension, and especially on coral-reefs; and had he reflected on the effects of long-continued, gentle winds, in drifting up the finer particles, he would hardly have advanced the argument of quantity, which is seldom trustworthy in geology. If the calcareous matter has originated from disintegrated shells and corals, the presence of animal matter is what might have been expected. Mr. Anderson analysed for Dr. Macaulay part of a cast, and he found it composed of:— Carbonate of lime......73.15 Silica.................11.90 Phosphate of lime.......8.81 Animal matter...........4.25 Sulphate of lime......a trace 98.11)

Calcareous deposits, like these of King George's Sound, are of vast extent on the Australian shores. Dr. Fitton remarks, that "recent calcareous breccia (by which term all these deposits are included) was found during Baudin's voyage, over a space of no less than twenty-five degrees of latitude and an equal extent of longitude, on the southern, western, and north-western coasts." (For ample details on this formation consult Dr. Fitton "Appendix to Captain King's Voyage." Dr. Fitton is inclined to attribute a concretionary origin to the branching bodies: I may remark, that I have seen in beds of sand in La Plata cylindrical stems which no doubt thus originated; but they differed much in appearance from these at Bald Head, and the other places above specified.) It appears also from M. Peron, with whose observations and opinions on the origin of the calcareous matter and branching casts mine entirely accord, that the deposit is generally much more continuous than near King George's Sound. At Swan River, Archdeacon Scott states that in one part it extends ten miles inland. ("Proceedings of the Geolog. Soc." volume 1 page 320.) Captain Wickham, moreover, informs me that during his late survey of the western coast, the bottom of the sea, wherever the vessel anchored, was ascertained, by crowbars being let down, to consist of white calcareous matter. Hence it seems that along this coast, as at Bermuda and at Keeling Atoll, submarine and subaerial deposits are contemporaneously in process of formation, from the disintegration of marine organic bodies. The extent of these deposits, considering their origin, is very striking; and they can be compared in this respect only with the great coral-reefs of the Indian and Pacific Oceans. In other parts of the world, for instance in South America, there are SUPERFICIAL calcareous deposits of great extent, in which not a trace of organic structure is discoverable; these observations would lead to the inquiry, whether such deposits may not, also, have been formed from disintegrated shells and corals.

CAPE OF GOOD HOPE.

After the accounts given by Barrow, Carmichael, Basil Hall, and W.B. Clarke of the geology of this district, I shall confine myself to a few observations on the junction of the three principal formations. The fundamental rock is granite (In several places I observed in the granite, small dark-coloured balls, composed of minute scales of black mica in a tough basis. In another place, I found crystals of black schorl radiating from a common centre. Dr. Andrew Smith found, in the interior parts of the country, some beautiful specimens of granite, with silvery mica radiating or rather branching, like moss, from central points. At the Geological Society, there are specimens of granite with crystallised feldspar branching and radiating in like manner.), overlaid by clay-slate: the latter is generally hard, and glossy from containing minute scales of mica; it alternates with, and passes into, beds of slightly crystalline, feldspathic, slaty rock. This clay-slate is remarkable from being in some places (as on the Lion's Rump) decomposed, even to the depth of twenty feet, into a pale-coloured, sandstone-like rock, which has been mistaken, I believe, by some observers, for a separate formation. I was guided by Dr. Andrew Smith to a fine junction at Green Point between the granite and clay-slate: the latter at the distance of a quarter of a mile from the spot, where the granite appears on the beach (though, probably, the granite is much nearer underground), becomes slightly more compact and crystalline. At a less distance, some of the beds of clay-slate are of a homogeneous texture, and obscurely striped with different zones of colour, whilst others are obscurely spotted. Within a hundred yards of the first vein of granite, the clay-slate consists of several varieties; some compact with a tinge of purple, others glistening with numerous minute scales of mica and imperfectly crystallised feldspar; some obscurely granular, others porphyritic with small, elongated spots of a soft white mineral, which being easily corroded, gives to this variety a vesicular appearance. Close to the granite, the clay-slate is changed into a dark-coloured, laminated rock, having a granular fracture, which is due to imperfect crystals of feldspar, coated by minute, brilliant scales of mica.

The actual junction between the granitic and clay-slate districts extends over a width of about two hundred yards, and consists of irregular masses and of numerous dikes of granite, entangled and surrounded by the clay- slate: most of the dikes range in a N.W. and S.E. line, parallel to the cleavage of the slate. As we leave the junction, thin beds, and lastly, mere films of the altered clay-slate are seen, quite isolated, as if floating, in the coarsely crystallised granite; but although completely detached, they all retain traces of the uniform N.W. and S.E. cleavage. This fact has been observed in other similar cases, and has been advanced by some eminent geologists (See M. Keilhau "Theory on Granite" translated in the "Edinburgh New Philosophical Journal" volume 24 page 402.), as a great difficulty on the ordinary theory, of granite having been injected whilst liquified; but if we reflect on the probable state of the lower surface of a laminated mass, like clay-slate, after having been violently arched by a body of molten granite, we may conclude that it would be full of fissures parallel to the planes of cleavage; and that these would be filled with granite, so that wherever the fissures were close to each other, mere parting layers or wedges of the slate would depend into the granite. Should, therefore, the whole body of rock afterwards become worn down and denuded, the lower ends of these dependent masses or wedges of slate would be left quite isolated in the granite; yet they would retain their proper lines of cleavage, from having been united, whilst the granite was fluid, with a continuous covering of clay-slate.

Following, in company with Dr. A. Smith, the line of junction between the granite and the slate, as it stretched inland, in a S.E. direction, we came to a place, where the slate was converted into a fine-grained, perfectly characterised gneiss, composed of yellow-brown granular feldspar, of abundant black brilliant mica, and of few and thin laminae of quartz. From the abundance of the mica in this gneiss, compared with the small quantity and excessively minute scales, in which it exists in the glossy clay-slate, we must conclude, that it has been here formed by the metamorphic action—a circumstance doubted, under nearly similar circumstances, by some authors. The laminae of the clay-slate are straight; and it was interesting to observe, that as they assumed the character of gneiss, they became undulatory with some of the smaller flexures angular, like the laminae of many true metamorphic schists.

SANDSTONE FORMATION.

This formation makes the most imposing feature in the geology of Southern Africa. The strata are in many parts horizontal, and attain a thickness of about two thousand feet. The sandstone varies in character; it contains little earthy matter, but is often stained with iron; some of the beds are very fine-grained and quite white; others are as compact and homogeneous as quartz rock. In some places I observed a breccia of quartz, with the fragments almost dissolved in a siliceous paste. Broad veins of quartz, often including large and perfect crystals, are very numerous; and it is evident in nearly all the strata, that silica has been deposited from solution in remarkable quantity. Many of the varieties of quartzite appeared quite like metamorphic rocks; but from the upper strata being as siliceous as the lower, and from the undisturbed junctions with the granite, which in many places can be examined, I can hardly believe that these sandstone-strata have been exposed to heat. (The Rev. W.B. Clarke, however, states, to my surprise ("Geolog. Proceedings" volume 3 page 422), that the sandstone in some parts is penetrated by granitic dikes: such dikes must belong to an epoch altogether subsequent to that when the molten granite acted on the clay-slate.) On the lines of junction between these two great formations, I found in several places the granite decayed to the depth of a few inches, and succeeded, either by a thin layer of ferruginous shale, or by four or five inches in thickness of the re-cemented crystals of the granite, on which the great pile of sandstone immediately rested.

Mr. Schomburgk has described ("Geographical Journal" volume 10 page 246.) a great sandstone formation in Northern Brazil, resting on granite, and resembling to a remarkable degree, in composition and in the external form of the land, this formation of the Cape of Good Hope. The sandstones of the great platforms of Eastern Australia, which also rest on granite, differ in containing more earthy and less siliceous matter. No fossil remains have been discovered in these three vast deposits. Finally, I may add that I did not see any boulders of far-transported rocks at the Cape of Good Hope, or on the eastern and western shores of Australia, or at Van Diemen's Land. In the northern island of New Zealand, I noticed some large blocks of greenstone, but whether their parent rock was far distant, I had no opportunity of determining.

INDEX TO VOLCANIC ISLANDS.

Abel, M., on calcareous casts at the Cape of Good Hope.

Abingdon island.

Abrolhos islands, incrustation on.

Aeriform explosions at Ascension.

Albatross, driven from St. Helena.

Albemarle island.

Albite, at the Galapagos archipelago.

Amygdaloidal cells, half filled.

Amygdaloids, calcareous origin of.

Ascension, arborescent incrustation on rocks of. -absence of dikes, freedom from volcanic action, and state of lava-streams.

Ascidia, extinction of.

Atlantic Ocean, new volcanic focus in.

Augite, fused.

Australia.

Azores.

Bahia in Brazil, dikes at.

Bailly, M., on the mountains of Mauritius.

Bald Head.

Banks' Cove.

Barn, The, St. Helena.

Basalt, specific gravity of.

Basaltic coast-mountains at Mauritius. -at St. Helena. -at St. Jago.

Beaumont, M. Elie de, on circular subsidences in lava. -on dikes indicating elevation. -on inclination of lava-streams. -on laminated dikes.

Bermuda, calcareous rocks of.

Beudant, M., on bombs. -on jasper. -on laminated trachyte. -on obsidian of Hungary. -on silex in trachyte.

Bole.

Bombs, volcanic.

Bory St. Vincent, on bombs.

Boulders, absence in Australia and Cape of Good Hope.

Brattle island.

Brewster, Sir D., on a calcareo-animal substance. -on decomposed glass.

Brown, Mr. R., on extinct plants from Van Diemen's land. -on sphaerulitic bodies in silicified wood.

Buch, Von, on cavernous lava. -on central volcanoes. -on crystals sinking in obsidian. -on laminated lava. -on obsidian streams. -on olivine in basalt. -on superficial calcareous beds in the Canary islands.

Calcareous deposit at St. Jago affected by heat. -fibrous matter, entangled in streaks in scoriae. -freestone at Ascension. -incrustations at Ascension. -sandstone at St. Helena. -superficial beds at King George's sound.

Cape of Good Hope.

Carbonic acid, expulsion of, by heat.

Carmichael, Capt., on glassy coatings to dikes.

Casts, calcareous, of branches.

Chalcedonic nodules.

Chalcedony in basalt and in silicified wood.

Chatham island.

Chlorophaeite.

Clarke, Rev. W., on the Cape of Good Hope.

Clay-slate, its decomposition and junction with granite at the Cape of Good Hope.

Cleavage of clay-slate in Australia.

Cleavage, cross, in sandstone.

Coast denudation at St. Helena.

Columnar basalt.

"Comptes Rendus," account of volcanic phenomena in the Atlantic.

Concepcion, earthquake of.

Concretions in aqueous and igneous rocks compared. -in tuff. -of obsidian.

Conglomerate, recent, at St. Jago.

Coquimbo, curious rock of.

Corals, fossil, from Van Diemen's Land.

Crater, segment of, at the Galapagos. -great central one at St. Helena. -internal ledges round, and parapet on.

Craters, basaltic, at Ascension. -form of, affected by the trade wind. -of elevation. -of tuff at Terceira. -of tuff at the Galapagos archipelago. -their breached state. -small basaltic at St. Jago. —at the Galapagos archipelago.

Crystallisation favoured by space.

Dartigues, M., on sphaerulites.

Daubeny, Dr., on a basin-formed island. -on fragments in trachyte.

D'Aubuisson on hills of phonolite. -on the composition of obsidian. -on the lamination of clay-slate.

De la Beche, Sir H., on magnesia in erupted lime. -on specific gravity of limestones.

Denudation of coast at St. Helena.

Diana's Peak, St. Helena.

Dieffenbach, Dr., on the Chatham Islands.

Dikes, truncated, on central crateriform ridge of St. Helena. -at St. Helena; number of; coated by a glossy layer; uniform thickness of. -great parallel ones at St. Helena. -not observed at Ascension. -of tuff. -of trap in the plutonic series. -remnants of, extending far into the sea round St. Helena.

Dislocations at Ascension. -at St. Helena.

Distribution of volcanic islands.

Dolomieu, on decomposed trachyte. -on laminated lava. -on obsidian.

Dree, M., on crystals sinking in lava.

Dufrenoy, M., on the composition of the surface of certain lava-streams. -on the inclination of tuff-strata.

Eggs of birds embedded at St. Helena. -of turtle at Ascension.

Ejected fragments at Ascension. -at the Galapagos archipelago.

Elevation of St. Helena. -the Galapagos archipelago. -Van Diemen's Land, Cape of Good Hope, New Zealand, Australia, and Chatham island. -of volcanic islands.

Ellis, Rev. W., on ledges within the great crater at Hawaii. -on marine remains at Otaheite.

Eruption, fissures of.

Extinction of land-shells at St. Helena.

Faraday, Mr., on the expulsion of carbonic acid gas.

Feldspar, fusibility of. -in radiating crystals. -Labrador, ejected.

Feldspathic lavas. -at St. Helena. -rock, alternating with obsidian. -lamination, and origin of.

Fernando Noronha.

Ferruginous superficial beds.

Fibrous calcareous matter at St. Jago.

Fissures of eruption.

Fitton, Dr., on calcareous breccia.

Flagstaff Hill, St. Helena.

Fleurian de Bellevue on sphaerulites.

Fluidity of lavas.

Forbes, Professor, on the structure of glaciers.

Fragments ejected at Ascension. -at the Galapagos archipelago.

Freshwater Bay.

Fuerteventura (Feurteventura), calcareous beds of.

Galapagos archipelago. -parapets round craters.

Gay Lussac, on the expulsion of carbonic acid gas.

Glaciers, their structure.

Glossiness of texture, origin of.

Gneiss, derived from clay-slate. -with a great embedded fragment.

Gneiss-granite, form of hills of.

Good Hope, Cape of.

Gorges, narrow, at St. Helena.

Granite, junction with clay-slate, at the Cape of Good Hope.

Granitic ejected fragments.

Gravity, specific, of lavas.

Gypsum, at Ascension. -in volcanic strata at St. Helena. -on surface of the ground at ditto.

Hall, Sir J., on the expulsion of carbonic acid gas.

Heat, action of, on calcareous matter.

Hennah, Mr., on ashes at Ascension.

Henslow, Prof., on chalcedony.

Hoffmann, on decomposed trachyte.

Holland, Dr., on Iceland.

Horner, Mr., on a calcareo-animal substance. -on fusibility of feldspar.

Hubbard, Dr., on dikes.

Humboldt on ejected fragments. -on obsidian formations. -on parapets round craters. -on sphaerulites.

Hutton on amygdaloids.

Hyalite in decomposed trachyte.

Iceland, stratification of the circumferential hills.

Islands, volcanic, distribution of. -their elevation.

Incrustation, on St. Paul's rocks.

Incrustations, calcareous, at Ascension.

Jago, St.

James island.

Jasper, origin of.

Jonnes, M. Moreau de, on craters affected by wind.

Juan Fernandez.

Keilhau, M., on granite.

Kicker Rock.

King George's sound.

Labrador feldspar, ejected.

Lakes at bases of volcanoes.

Lamination of volcanic rocks.

Land-shells, extinct, at St. Helena.

Lanzarote, calcareous beds of.

Lava, adhesion to sides of a gorge. -feldspathic. -with cells semi-amygdaloidal.

Lavas, specific gravity of.

Lava-streams blending together at St. Jago. -composition of surface of. -differences in the state of their surfaces. -extreme thinness of. -heaved up into hillocks at the Galapagos archipelago. -their fluidity. -with irregular hummocks at Ascension.

Lead, separation from silver.

Lesson, M., on craters at Ascension.

Leucite.

Lime, sulphate of, at Ascension.

Lonsdale, Mr., on fossil-corals from Van Diemen's land.

Lot, St. Helena.

Lyell, Mr., on craters of elevation. -on embedded turtles' eggs. -on glossy coating to dikes.

Macaulay, Dr., on calcareous casts at Madeira.

MacCulloch, Dr., on an amygdaloid. -on chlorophaeite. -on laminated pitchstone.

Mackenzie, Sir G., on cavernous lava-streams. -on glossy coatings to dikes. -on obsidian streams. -on stratification in Iceland.

Madeira, calcareous casts at.

"Magazine, Nautical," account of volcanic phenomena in the Atlantic.

Marekanite.

Mauritius, crater of elevation of.

Mica, in rounded nodules. -origin in metamorphic slate. -radiating form of.

Miller, Prof., on ejected Labrador feldspar. -on quartz crystals in obsidian beds.

Mitchell, Sir T., on bombs. -on the Australian valleys.

Mud streams at the Galapagos archipelago.

Narborough island.

Nelson, Lieut., on the Bermuda islands.

New Caledonia.

New Red sandstone, cross cleavage of.

New South Wales.

New Zealand.

Nulliporae (fossil), resembling concretions.

Obsidian, absent at the Galapagos archipelago. -bombs of. -composition and origin of. -crystals of feldspar sink in. -its irruption from lofty craters. -passage of beds into. -specific gravity of. -streams of.

Olivine decomposed at St. Jago. -at Van Diemen's land. -in the lavas at the Galapagos archipelago.

Oolitic structure of recent calcareous beds at St. Helena.

Otaheite.

Oysters, extinction of.

Panza islands, laminated trachyte of.

Pattinson, Mr., on the separation of lead and silver.

Paul's, St., rocks of.

Pearlstone.

Peperino.

Peron, M., on calcareous rocks of Australia.

Phonolite, hills of. -laminated. -with more fusible hornblende.

Pitchstone. -dikes of.

Plants, extinct.

Plutonic rocks, separation of constituent parts of, by gravity.

Porto Praya.

Prevost, M. C., on rarity of great dislocations in volcanic islands.

Prosperous hill, St. Helena.

Pumice, absent at the Galapagos archipelago. -laminated.

Puy de Dome, trachyte of.

Quail island, St. Jago.

Quartz, crystals of, in beds alternating with obsidian. -crystallised in sandstone. -fusibility of. -rock, mottled from metamorphic action with earthy matter.

Red hill.

Resin-like altered scoriae.

Rio de Janeiro, gneiss of.

Robert, M., on strata of Iceland.

Rogers, Professor, on curved lines of elevation.

Salses, compared with tuff craters.

Salt deposited by the sea. -in volcanic strata. -lakes of, in craters.

Sandstone of Brazil. -of the Cape of Good Hope. -platforms of, in New South Wales.

Schorl, radiating.

Scrope, Mr. P., on laminated trachyte. -on obsidian. -on separation of trachyte and basalt. -on silex in trachyte. -on sphaerulites.

Seale, Mr., geognosy of St. Helena. -on dikes. -on embedded birds' bones.

Seale, on extinct shells of St. Helena.

Sedgwick, Professor, on concretions.

Septaria, in concretions in tuff.

Serpulae on upraised rocks.

Seychelles.

Shells, colour of, affected by light. -from Van Diemen's land. -land, extinct, at St. Helena. -particles of, drifted by the wind at St. Helena.

Shelly matter deposited by the waves.

Siau, M., on ripples.

Signal Post Hill.

Silica, deposited by steam. -large proportion of, in obsidian. -specific gravity of.

Siliceous sinter.

Smith, Dr. A., on junction of granite and clay-slate.

Spallanzani on decomposed trachyte.

Specific gravity of recent calcareous rocks and of limestone. -of lavas.

Sphaerulites in glass and in silicified wood. -in obsidian.

Sowerby, Mr. G.B., on fossil-shells from Van Diemen's land. -from St. Jago. -land-shells from St. Helena.

St. Helena. -crater of elevation of.

St. Jago, crater of elevation of. -effects of calcareous matter on lava.

St. Paul's rocks.

Stokes, Mr., collections of sphaerulites and of obsidians.

Stony-top, Little. -Great.

Stratification of sandstone in New South Wales.

Streams of obsidian.

Stutchbury, Mr., on marine remains at Otaheite.

Subsided space at Ascension.

Tahiti.

Talus, stratified, within tuff craters.

Terceira.

Tertiary deposit of St. Jago.

Trachyte, absent at the Galapagos archipelago. -at Ascension. -at Terceira. -decomposition of, by steam. -its lamination. -its separation from basalt. -softened at Ascension. -specific gravity of. -with singular veins.

Trap-dikes in the plutonic series. -at King George's sound.

Travertin at Van Diemen's land.

Tropic-bird, now rare, at St. Helena.

Tuff, craters of. -their breached state. -peculiar kind of.

Turner, Mr., on the separation of molten metals.

Tyerman and Bennett on marine remains at Huaheine.

Valleys, gorge-like, at St. Helena. -in New South Wales. -in St. Jago.

Van Diemen's land.

Veins in trachyte. -of jasper.

Vincent, Bory St., on bombs.

Volcanic bombs. -island in process of formation in the Atlantic. -islands, their distribution.

Wacke, its passage into lava.

Wackes, argillaceous.

Webster, Dr., on a basin-formed island. -on gypsum at Ascension.

White, Martin, on soundings.

Wind, effects of, on the form of craters.

THE END