more than 800 geographical miles in extent, rested on a sheet of fluid matter, are applicable, with nearly equal force, to the areas beneath the other trains." After enumerating several considerations, he continues: "It appears to me that there is little hazard in assuming, that this large portion of the earth's crust floats in a like manner on a sea of molten rock. Moreover, when we think of the increasing temperature of the strata, as we penetrate downwards in all parts of the world, and of the certainty that every portion of the surface rests on rocks which have once been liquefied ;-when we consider the multitude of points from which fluid rock is annually emitted, and the still greater number of points from which it has been emitted during the few last geological periods inclusive, which, as far as regards the cooling of the rock in the lowest abysses, may probably be considered as one, from the extreme slowness with which heat can escape from such depths ;--when we reflect how many and wide areas in all parts of the world are certainly known some to have been rising and others sinking, during the recent æra even to the present day, and do not forget the intimate connexion which has been shown to exist between these movements and the propulsion of liquefied rock to the surface in the volcano ;- we are urged to include the entire globe in the foregoing hypothesis.--The furthest generalization, which the consideration of the volcanic phenomena appears to lead to, is that the configuration of the fluid surface of the earth's nucleus is subject to some change,its cause completely unknown,— and intermittent, but irresistible." Pp. 629–631.

-its action slow

But justice to this subject requires it to be stated that Mr. William Hopkins has applied his pre-eminent talents in mathematical analysis to the solution of this problem. Of this laborious investigation he has given the processes and the results, in four papers, communicated to the Royal Society in the years 1838, 1839, and 1842, "On the State of the Interior of the Earth ;" and published in the Philos. Trans. of those years. That the entire mass of the earth was originally in a fluid state, and that, by cooling, it has obtained a superficial coating of solidified matter, was first adopted as a result of astronomical considerations, but is now corroborated by the discoveries of geology. It occurred to Mr. H. that an indirect, but sure test of the truth of the hypothesis might be derived from the consideration of the Nutation of the Earth's Axis, a fact arising from the attractions of the sun and the moon, which produce the Precession of the Equinoxes. It is obvious that this nutation must affect the earth differently on the supposition of its being a fluid contained in a solid spheroidal shell, from what it would do if the globe were throughout a solid mass. Equally is it to be expected that differences of result will be produced according as the quantity of the contained fluid is less or

greater; that is, as the thickness of the containing crust is greater or less. An indefinite number of conditions is thus presented for selection and investigation. To the immense labour of these operations, Mr. Hopkins applied himself, with his characteristic power and perseverance. Now the amount of the gyratory change constantly produced by the nutation of the pole is astronomically known. But the amount, as deduced from the hypothesis of the earth's being composed of a heterogeneous solid shell inclosing a heterogeneous fluid, will not agree with the actually known amount, unless the ellipticity of the interior surface of the shell were less by a certain quantity than that of the outward surface. Finally, Mr. H. arrives at the conclusion that the thickness of the crust, to answer all the conditions, must be at least equal to one-fourth or one-fifth of the distance from the outer circumference to the centre of the earth; in other words, that the thickness of the solid envelope of the globe cannot be less than 800 or 1000 miles.

Here then is an astounding difference from the conclusion before mentioned, that in which so many eminent and accomplished physicists agree, and of which the evidence appears unexceptionable. But Mr. H. has furnished a mode of conciliation. He supposes that the solidity of the crust is not everywhere the same, but that there are insulated fluid masses, or what may be called vesicles, interspersed through it, containing portions of more fusible matter, and which is actually in a state of fusion, forming subterranean reservoirs or lakes, some of which are distinct, and others communicating by passages of different degrees of openness or obstruction. This he proposes for the explanation of the phenomena of volcanos. We must extend the idea, by supposing an extensive distribution of those fiery lakes, so as to cause the observed ratio of the increase of temperature in the descent of mines.

This investigation has produced another very important result, in demonstrating that no change has taken place in the direction of the earth's axis, from the epoch of the formation of the external crust.

Fourth ed.-On this subject, at the sitting of the Paris Academy of Sciences, Dec. 9, 1844, "M. Elie de Beaumont made some observations on a question submitted for consideration, viz. What relation exists between the progressive cooling of the earth and that of its surface? M. de B. thinks that the experiments made by M. Arago, in the gardens of the Observatory at Paris, with thermometers sunk in the earth at various depths, furnish the most essential elements which are necessary for the solution of the problem. According to this solution, the antiquity of the period, when our globe was entirely incandescent, is of a remoteness which defies calculation." Athenæum, Dec. 28, 1844; p. 1202.

[C.]

Referred to at page 43.

ON THE NUMBER OF SPECIES IN THE EARLIER FOSSILIFEROUS ROCKS.

THIS proposition, as it stood in the first edition, expressed the doctrine, which till lately was received among geologists, that the organic remains found in the earliest rocks possessing any such remains at all, are "the fewest" in proportionate numbers. But the recent herculean toils of Mr. Murchison have opened new fields of view upon this interesting subject. The proposition is a little altered, that it may be in accordance with the observation of that distinguished geologist; "Another remarkable fact illustrating this point of inquiry is, that although the older fossiliferous strata often contain vast quantities of organic remains, the number of species is much smaller than in more recent deposits." Silur. Syst. p. 583.

It is interesting to observe the careful and cautious progress of Geology, as in other respects, so in this. Seven years before the publication of Mr. Murchison's work, Sir Henry De la Beche had treated this subject with his characteristic judgment and sagacity. I will cite a few paragraphs, as specimens of the penetration and anticipations of his geological mind, while the facts of the case were not as yet brought clearly to light; and with a wish also to excite my readers to peruse the whole passage. It must be premised that the German term grauwacke, now generally disused, must be understood as the same with, or including, the Silurian System.

Although, when we regard the mass of the grauwacke rocks, we are struck with the minute proportion that organic remains bear to the whole, we must still perceive that the atmosphere was capable of supporting vegetation; and the seas of sustaining zoophytes, crinoidea, annulata, conchifera, mollusca, crustacea, and fish. What other creatures existed, we are unable, from the absence of their remains, to judge it may however be by no means unphilosophical to conclude that vegetation did not exist alone on dry land, but that, consistently with the general harmony of nature, it afforded food to terrestrial creatures suited to the circumstances under which they were placed." [Yet no vestiges of such creatures have been found.]—"Whatever the kind of animal life may have been which first appeared on the surface of our planet, we may be certain that it was consistent with the wisdom and design which has always prevailed throughout nature; and that each creature was peculiarly adapted to that situation designed to be occupied by it." Geological Manual; third ed. pp. 428, 429; 1833.

4

I hope I shall not give pain to a very kind friend whose qualifications make him one of the safest authorities, by transcribing a part of a letter with which he has favoured me. The certainty and accuracy of the remarks may be fully relied upon, and their application is extensively important.

"In the older fossiliferous rocks, animal life appears in as full a developement with respect to SIZE, as in the existing analogous animals. The zoophagous cephalopods were also of gigantic growth. It does not appear that animal life, at that period, was limited with respect to NUMBER. The lowest Silurian rocks are crowded, in some localities, with organic bodies; and their absence over extensive districts is only a condition in the distribution of testacea, &c. which prevails in our seas. There are many coasts from which a reasoner, who had visited no other, might infer that marine animals are now few and smallsized; and, if he were a geologist, he might say, Life is on the decrease!" WILLIAM LONSDALE, ESQ. Sept. 7, 1839.

[On this subject, however, the following statements from men of the highest authority, as to both science and judgment, cannot be disregarded. But a conciliation arises from considering that the actually occurring fossils in the Protozoic rocks are no index of those which existed in previous periods of life; for causes of decomposition must have existed to a great extent. Not only countless individuals but vast groups, animal and vegetable, must have been totally obliterated. Mr. Lonsdale's doctrine, with regard to the lowest Silurian, is confirmed by the observations of the geologist of New York, Mr. James Hall, in the Boston Journal of Nat. Hist. Jan. 1845, p. 14.] "One general conclusion becomes very apparent wherever we meet with rocks admitting of the preservation of organic remains, the number of these decreases as we descend in the series, till we arrive at a period when the physical monuments of the globe bear no trace of organized beings; an abyss which gives no record of life, and which sets a bound to our zoological inquiries." Prof. Sedgwick, at the Plymouth Meeting of the Brit. Assoc. Aug. 3, 1841.

"These remains of life are most plentiful in the upper strata ; decrease in number and variety as we proceed downwards; and, by gradual and continual diminution, approach in the lowest to absolute extinction." Mr. Phillips, at the same time and place.

["It is now a truth which I consider as proved, that the ensemble of organized beings was RENEWED, not only in the interval of each of the great geological divisions which we have agreed to term Formations, but also at the time of the deposition of each particular member of all the formations. For example, I think that I can prove that, in the Oolitic formation (at least within the limits of the Swiss Jura) the organic contents of the Lias, those of the Oolitic group properly

T

so called, those of the Oxfordian group, and those of the Portlandian group, (as they occur in Switzerland,) are as different from each other as the fossils of the Lias from those of the Keuper [Pacilitic], or those of the Portlandian beds from those of the Neocomian formation [Greensand]. I also believe very little in the genetic descent of living species from those of the various tertiary layers which have been regarded as identical, but which, in my opinion, are specifically distinct. I cannot admit the idea of the transformation of species from one formation to another. In advancing these general notions, I do not wish to offer them as inductions drawn from the study of any particular class of animals, (of the fishes, for instance,) and [thence, by mere theoretical analogy,] applied to other classes; but as the results of direct observation of very considerable collections of fossils, of different formations, and belonging to different classes of animals, in the investigation of which I have been specially engaged for many years." Prof. Agassiz; Report to the Brit. Assoc. on the Fossil Fish of the Old Red; 1842.]

[D.]

Referred to at page 50, but intended to apply to all the Propositions of Lecture II.

SYNOPTIC TABLE OF STRATIFIED FORMATIONS.

In compliance with the desire of many and those unconnected friends, I have compiled a short view of the Strata of the Earth, almost exclusively as they are developed in our own island. But a similarity will apply to every other country, if the comparison be rightly made; for there are few formations known to exist in any part of the globe, which have not their equivalent or representative in Great Britain; excepting some of the numerous and circumscribed Tertiaries of the Continent.

As this attempt represents only the strata, or aqueous deposits, the reader will understand the vast mass of granite and other igneous rocks as lying under the gneiss, the last stratum; and will recollect that, in various forms, it has elevated, pierced through, and surmounted the strata. (See p. 37.) For impressing this lesson, he will not neglect to avail himself of the graphic representations presented in the splendid first plate of Dr. Buckland's Bridgewater Treatise; or De la Beche's Tabular View, which extends only to the Devonian; or the seventh of Dr. Mantell's Wonders of Geology; or the French plate by Viguier and Collon, as an Exposition of Alex. Brongniart's volume, Tableau des Terrains; or Bartlett's Index