except the coral are igneous; and the coral may rest, as we have reason to believe, on an igneous base. It is, therefore, a just conclusion, that the areas of the surface constituting the continents were first free from eruptive fires. These portions cooled first, and consequently the contraction in progress affected most the other parts. The great depressions occupied by the oceans thus began; and for a long period afterward, continued deepening by slow, though it may have been unequal, progress. This may be deemed a mere hypothesis; if so, it is not as groundless as the common assumption, that the oceans may have once been dry land, a view often the basis of geological reasoning. Let us look farther at the facts. Before the depression of the oceanic part of our globe had made much progress, the depth would be too shallow to contain the seas, and consequently the whole land would be under water. Is it not a fact that, in the early Silurian epoch, nearly every part of the globe was beneath the ocean? So we are taught by the extent of the formation. The depth of water over the continental portions would be very various; but those parts which now abound in the relics of marine life, were probably comparatively shallow, as amount of pressure, light, and dissolved air, are the principal circumstances influencing the distribution of animals in depth, and acted formerly, we may believe, as at the present period. Here, then, we see reason for what has been considered a most improbable supposition, the existence of an immense area covered in most parts by shallow seas, and so fitted for marine life. If we follow the progress of the land, we find that with each great epoch there has been a retiring of the sea. In the coal deposits we have an abundant land vegetation. Subsequently, the progress on the whole was giving increased extent and height to the land, and diminishing the area of the waters. Instead, therefore, of a bodily lifting of the continents to produce the apparent elevation, it may actually have been a retreating of the waters through the sinking of the ocean's bottom. The process, however, has not been a continuous one; for during each epoch-the Silurian and the more recent-there have been subsidences as well as seem ing and actual elevations, and various oscillations of the continental surface, from subaerial to submarine, and the reverse. When contraction had once taken place over the continents, as well as under the ocean, there may have afterwards been expansions again through the return of heat from some cause. And thus various irregularities have taken place, such as the rocks indicate. In the tertiary period, and since, the apparent rise of the land has been still to some extent in progress. And is there any evidence that this could have arisen from a sinking of the ocean's bed? The evidence is undoubted. For Mr Darwin has shewn satisfactorily (and farther observations to the same end, and to many interesting conclusions, will be presented in the writer's Geological Report on the Pacific), that a subsidence of some thousands of feet has taken place since the corals commenced their growth. Every coral island is a register of this subsidence.* And why should not the ocean's bottom subside, as well as the land? What has given the continental portions of our globe their elevation, as compared with other parts, if not the unequal contraction of the whole? Can we safely affirm -in words of high authority-" that the stability of the sea and the mobility of the land are demonstrated truths in geology,”† when mobile land forms also the bed of the ocean, and its changes must affect the stability of the superincumbent waters;-I ask, can we safely make this affirmation, until we know something more certain than past investigations have revealed, about the geological history of the two-thirds of the surface of our planet that are concealed beneath its oceans? In our conclusions from the above reasoning, we fall in nearly with the views presented by a distinguished French geologist, M. C. Prevost, who has argued with much force in favour of subsidence as a cause of the apparent elevation of the land; though it may be right to state, that these conclusions were arrived at previously to seeing his memoir.* There * See Silliman's Journal, xlv., p. 131, 1843. † Leonard Horner, Esq., Anniversary Address before the Geological Society of London, January, 1846; Quarterly Jour. of the Geol. Soc., No. 6, p. 199. *The general theory of changes of level by contraction and expansion, and ippear to be many objections to the opinions of M. Prevost, is they are expressed by him, inasmuch as no allowance is made or admitted for minor disturbances and actual elevations by subterranean forces. His views, however, are well worthy the attention of the geological inquirer. The principles explained place the general theory of change of level by contraction upon something better than a hypothetical basis, and are believed to explain the actual causes by which the changes have been produced. They correspond, moreover, with the view that ruptures, elevations, foldings, and contortions of strata have been produced in the course of contraction. The greater subsidence of the oceanic parts would necessarily occasion that lateral pressure required for the rise and various foldings of the Alleganies and like regions. A brief Review of the Classification of the Sedimentary Rocks of Cornwall. By Sir RODERICK IMPEY MURCHISON, G.C. St S., F.R.S., V.P.G.S. & R. Geogr. S., &c., &c. (In a Letter, addressed Sir C. LEMON, Bt., M.P.) Communicated for the Journal by the Author. MY DEAR SIR CHARLES,-In compliance with the promise I made when attending your last anniversary meeting at Penzance, I now the rise thus of continents, was first presented by Mr Babbage and De la Beche. M. C. Prevost takes the different ground, that all seeming elevations are the result of subsidence. His propositions are as follows (Bulletin de la Soc. Geol. de France, xi., 1839 à 1840, p. 186) : "1. Que le relief de la surface du sol est le résultat de grands affaissements successifs, qui, par contre-coup, et d'une manière secondaire, ont pu occasioner accidentellement des élévations absolues, des pressions latérales, des ploiements, des plissements, des ruptures, des tassements, des failes, etc.; mais que rien n'autorise à croire que ces divers accidents ont été produits par une cause agissant sous le sol, c'est-à-dire par une force soulevante ; "2. Que les dislocations du sol sont des effets complexes de retrait, de contraction, de plissement et de chute;" "3. Que les matières ignées (granites, porphyres, trachytes, basaltes, lavas), loin d'avoir soulevé et rompu le sol pour s'echapper, ont seulement profité des solutions de continuité qui leur ont été offertes par le retrait et les ruptures, pour sortir, suinter et s'épancher au-dehors." VOL. XLIII. NO. LXXXV.—JULY 1847. C give you a more decided opinion than I was then enabled to do, respecting the age of the lowest and oldest of the sedimentary rocks of Cornwall. Not having seen the fossils collected by Mr Peach on the south coast of Cornwall, I then found it difficult to come to any other conclusion than that at which Professor Sedgwick and myself had long ago arrived; viz., that, with the exception of the presence, in the north-eastern extremity of the country, of a portion of the culmiferous (carboniferous) trough of central Devon, the remaining and underlying strata of Cornwall were of the age of the Devonian or old Red system. The few Cornish fossils which were then shewn to me in your museum, were unquestionably similar to those with which I was formerly familiar in Devonshire and North Cornwall, as well as with those of the Rhenish provinces and the Eifel, which Professor Sedgwick and myself had shewn to occupy a like geological position. They were in fact, forms of the same type as those which, at the suggestion of Mr Lonsdale, and with the assistance of Mr James Sowerby on one occasion, and with the help of MM. de Verneuil and d'Archiac on another,† we had published as characteristic of a group of intermediate characters, pertaining to strata lying beneath the carboniferous rocks and above the Silurian system. In a word, they were identical with some of the numerous fossils of Devon and North Cornwall, published in the work of Professor Phillips; who, in pointing out in certain tracts the connection of this group with the carboniferous fossils, which he had so well described, and in others with the Silurian forms I had published, had also concluded that the great mass of fossiliferous strata which rise up from beneath the culm measures of central Devon were of the same intermediate characters. In his valuable maps of Cornwall and Devon, Sir H. de la Beche gave essentially the same views of geological succession; and, lastly, in his report upon the geological structure of that region, he described certain detailed sections in the southern districts of Cornwall, to which I will presently advert. In proposing the word "Devonian," as applied to the intermediate strata in question, Professor Sedgwick and myself thus qualified our meaning in regard to the extension of such rocks into Cornwall:"In asserting that the stratified rocks of Devonshire and Cornwall are, upon a broad scale, the equivalents of the carboniferous and old red systems, we do not however deny, that, in certain tracts, the lowest members of some of these rocks may represent the upper division of the Silurian system; for although we have as yet found few if any of the fossils most typical of that system, we admit that when the sediments of a given epoch have been accumulated under peculiar *Trans. Geol. Soc., N.S., vol. v., p. 633. Paleozoic Fossils of Devon and Cornwall. conditions, we must expect to find considerable variations in the forms of animal life. Again, we know that the rocks of this region have undergone great changes in assuming their hard and slaty character; and under such circumstances, the difficulty of precisely limiting the boundary line of any portion of them is prodigiously increased.' The truth is, that neither Sir H. De la Beche and Professor Phillips, nor Professor Sedgwick and myself, had, at the time when our works were published, seen any fossils from South Cornwall sufficiently distinct to warrant the conclusion, that it contained forms of an older type than those which had been detected in North and South Devon, and in the west of Cornwall. It was, therefore, believed (and all geological maps were coloured accordingly) that the zone of rocks occupying the southern headlands of Cornwall, between the Bay of Plymouth on the east, and the Lizard Head on the west, were simply downward expansions of the fossiliferous "Devonian" strata. In this state of the question, your associate Mr Peach began his labours in collecting fossils along the southern headlands of Cornwall. He first ascertained that certain forms first discovered by Messrs Couch in the environs of Polperro were fishes, which he exhibited at the Cork Meeting of the British Association, and concerning which Professor Phillips and myself could only venture (so obscure did they appear to us) to give the guarded, though suggestive opinion, which Mr Peach has recorded in your thirtieth Report. I then ventured to surmise, that these ichthyolites might belong to the Upper Silurian rocks, the oldest in which the remains of any vertebrated animals had yet been discovered, because "they occurred in rocks forming the axis of South Devon and Cornwall, which I had always considered to be the oldest in that country." Iu pursuing his researches, Mr Peach published, in 1844, a synopsis of the Cornish fossils from various localities, in which, besides the ichthyolites of Polperro, he identified several mollusca from Gorran Haven, Caerhayes, and Carn Gorran Bay, with typical Silurian species. These were the fossils I was so anxious to see at Penzance; and Mr Peach having obligingly forwarded them to me in London, I no sooner unpacked the box, than I found that true Silurian and even Lower Silurian rocks existed in Cornwall,-the proofs being the presence of certain simple-plaited Orthida, which are invariably typical of that age. But although Mr Peach had come to a correct general conclusion, the specific names he attached to the South Cornish fossils in your thirtieth Report are not correct. In respect to the ichthyolites from the slates of Polperro, Pentuan, &c., they have been referred to our mutual friend Sir Philip Egerton, who his better versed in the classification of Agassiz than any of our countrymen, and he thus writes to me concerning them :- "These remains are *Phil. Mag., 1839, vol. xiv., p. 241. |