that required for simple igneous fusion. The intervention of water in giving liquidity to lavas, has, in fact, long been taught by Scrope, and, notwithstanding the opposition of Plutonists like Durocher, Fournet, and Rivière, is now very generally admitted. In this connection, the reader is referred to the GEOLOGICAL MAGAZINE for February, 1868, page 57, where the history of this question is discussed. It may here be remarked that if we regard the liquefaction of heated rocks under great pressure, and in presence of water, as a process of solution rather than of fusion, it would follow that diminution of pressure, as supposed by Mr. Scrope, would cause not liquefaction but the reverse. The mechanical pressure of great accumulations of sediment is to be regarded as co-operating with heat to augment the solvent action of the water, and as being thus one of the efficient causes of the liquefaction of deeply buried sedimentary rocks.

That water intervenes not only in the phenomena of volcanic eruptions, but in the crystallization of the minerals of eruptive rocks, which have been formed at temperatures far below that of igneous fusion, is a fact not easily reconciled with either the first or the second hypothesis of volcanic action, but is in perfect accordance with the one here maintained, which is also strongly supported by the study of the chemical composition of igneous rocks. These are generally referred to two great divisions, corresponding to what have been designated the trachytic and pyroxenic types, and to account for their origin, a separation of a liquid igneous mass beneath the earth's crust into two layers of acid and basic silicates, was imagined by Phillips, Durocher, and Bunsen. The latter, as is well known, has calculated the normal composition of these supposed trachytic and pyroxenic magmas, and conceives that from them, either separately, or by admixture, the various eruptive rocks are derived; so that the amounts of alumina, lime, magnesia, and alkalies, sustain a constant relation to the silica in the rock. If, however, we examine the analyses of the eruptive rocks of Hungary and Armenia made by Streng, and put forward in support of this view, there will be found such discrepancies between the actual and the calculated results as to throw grave doubts on Bunsen's hypothesis.

Two things become apparent from a study of the chemical nature of eruptive rocks; first, that their composition presents such variations as are irreconcilable with the simple origin generally assigned to them, and second, that it is similar to that of sedimentary rocks whose history and origin it is, in most cases, not difficult to trace. I have elsewhere pointed out how the natural operation of mechanical and chemical agencies tends to produce among sediments, a separation into two classes, corresponding to the two great divisions above noticed. From the mode of their accumulation, however, great variations must exist in the composition of the sediments corresponding to many of the varieties presented by eruptive rocks. The careful study of stratified rocks of aqueous origin discloses, in addition to these, the existence of deposits of basic

silicates of peculiar types. Some of these are in great part magnesian, others consist of compounds like anorthite and labradorite, highly aluminous basic silicates, in which lime and soda enter to the almost complete exclusion of magnesia and other bases; while in the masses of pinite or agalmatolite rock, we have a similar aluminous silicate, in which lime and magnesia are wanting, and potash is the predominent alkali. In such sediments as these just enumerated, we find the representatives of eruptive rocks like peridotite, phonolite, leucitophyre, and similar rocks which are so many exceptions in the basic group of Bunsen. As, however, they are represented in the sediments of the earth's crust, their appearance as exotic rocks, consequent upon a softening and extravasation of the more easily liquefiable strata of deeply buried formations, is readily and simply explained.1

The object of the present communication has been to call the attention of geologists to the neglected views of Keferstein and Herschel, which I have endeavoured to extend and to adapt to the present state of our knowledge. It is proposed in another paper to consider the question of the agencies which have regulated the geographical distribution of volcanic phenomena both in ancient and in modern times.

Montreal, Canada, March, 1869.

D

III.—NOTES ON CONTINENTAL GEOLOGY AND PALEONTOLOGY. BY THOMAS DAVIDSON, F.R.S., F.G.S.

(Continued from p. 205).
(PART III.)

URING my recent stay at Geneva the eminent geologists MM. F. J. Pictet and P. de Loriol showed me a very considerable number of fossils, which they had collected from the middle and lower portions of the Cretaceous system of Switzerland and Savoy, and carefully explained the position of the beds from which they had been obtained. M. Pictet subsequently, at my request, recorded in manuscript his most recent views in connection with this important topic, of which I will shortly reproduce a translation for the benefit of the readers of the GEOLOGICAL MAGAZINE. He has, however, restricted his table and explanations to the middle and lower portions of the Cretaceous system, because the regions which surround Geneva do not exhibit any representatives or evidence in connection with the upper stages. M. Pictet has also explained his views with reference to the rock which, at the Porte-de-France, contains the Terebratula janitor, and of the Carpathian or Stramberg limestone, which has been placed by M. Hébert at the base of the Cretaceous system, but which others have referred to the Jurassic epoch. The correct determination of the true age of these rocks is a subject of very great importance, since they contain a rich assem

See in this connection the Canadian Journal, for 1858, p. 203; Quart. Jour. Geol. Society for 1859, p. 494; Amer. Jour. Science [2] xxxvii., 255, xxxviii. 182; also Geology of Canada, 1863, pp. 643, 669, and Rep. Geol. Canada, 1866, p. 230.

blage of species bearing a particular and well-marked stamp, as may be seen by a glance at Professor Suess's admirable monograph, "Die Brachiopoden der Stramberger Schichten," as well as at those by Zettel, etc., on other classes.

of

In connection with this subject of the difficult classification of the rocks above mentioned, M. Lory, a distinguished French geologist, favourably known on account of his able researches in the geology of the French Alps, etc., has kindly transmitted to me a summary his recent labours which he communicated on the 3rd of May to the Geological Society of France, and of which I now offer a translation prior to introducing M. Pictet's valuable observations upon the same subject. I have, moreover, considered it absolutely necessary to introduce the discussion in connection with the T. viator limestone of the Port-de-France and that of Stramberg, as they are so intimately connected with the fossils to be assigned to the Cretaceous period.

Russet Brown Limestone, with Ostrea Russet brown Limestone, with Ostrea

gula.

Marls and Limestone, with Pterocera Corresponding to a complete change of fauna

M. Lory writes to me, on the 29th of April, "In all the papers where I have had occasion to refer to the beds with Tereb. diphya (or janitor, Pictet), of the Porte-de-France, of the lithographic limestone and of the hydraulic cement beds which lie above, either at the Porte-de-France, at Aizy, and other places in the neighbourhood of Grenoble, or at Lemenc, and other spots in the vicinity of Chambéry, I had always, until within the last few years, considered these beds as Jurassic. In 1851 I made known the fauna of the breccia of Aizy, as a last remnant of the extension of the coral limestone which terminated at the border of the Alpine mass (Bull. Soc. Geol. France, 2nd ser., vol. ix., p. 54), also the difficulty of tracing a limit between the limestone of the Porte-de-France and the base of the true Neocomien of the Alps (ibid. p. 52). I concluded that the Alpine mass had emerged above the sea after the deposition of the upper Oxford clay, before (or during) the deposition of the Coral rag, and subsequently replaced by a subsidence under the sea at the commencement only of the Neocomien period (ibid. p. 236, 238) an opinion adopted and maintained by M. Hébert, and upon which we were always agreed. I have since shown that the Neocomien deposits must have commenced in the Alpine region. before they spread in the Jurassic one: Since in the latter the first deposits were the Valanginian limestone represented at Grenoble, by the limestone of Fontanil, above which we find again the large deposit of the marls with small Ammonites, and with Belemnites latus of the Mediterranean region (Bull. Soc. Geol., vol. xv., p. 30), so that the Neocomien sea came from the South, after having formed this deposit, and extended itself over the Jura at the period of the deposition of the Valanginian limestone and later again in the Haute Savona where we find only the marls with Spatangus (Desc. Geol. of the Dauphiné, p. 159 and 189).

"In my opinion, and I believe in that of M. Hébert, this deposition of Lower Neocomien strata took place during the long epoch of intermission of the marine deposits in the Anglo-Parisian basin, represented by the fresh water deposits of the Wealden.

"In the last paper which I published on these beds, I still maintained the upper beds of the Porte-de-France to be of Jurassic age on account of the Ammonites which D'Orbigny had identified as A. anceps, viator, Hommairei, Adela, etc., occurring in beds, some immediately below, and others above the principal horizon of the Ter. diphya (Bull. 2nd ser., vol. xxiii., p. 516), and above all,

on account of the small fauna of the breccia of Aizy and Lemenc, which is believed to be incontestably Jurassic. Since then I have attentively re-examined the localities in order to furnish M. Pictet and M. Hébert with the stratigraphical data of which they were in need (neither the one nor the other having visited those localities in the neighbourhood of Grenoble), and I have arrived at the following conclusions: First, that the principal and lower mass of the limestone of the Porte-de-France contains a fauna which is entirely Oxfordian; it is terminated by a bed with large Aptychus (A. lævis and A. lamellosus) which form the bottom of the quarry of the Porte-de-France.

Secondly: In a bed of compact limestone situated above the last described, the T. janitor (or diphya) begins to appear along with the Ammonites under discussion, which M. Hébert considers referable to Neocomien types, and which do not in any case occur along with the Jurassic fauna underneath: moreover, no species bearing an evident Jurassic type has been met with in this zone. Above this, beds occur containing scarcely any other fossils save T. janitor with T. gratianopolitana (Pictet). These are followed (always in the ascending order) by others containing Ammonites, the lithographic limestones are more developed and richer in fossils at Aizy, and with them is intercalated, as a local accident, the breccia of Aizy, of Lemenc, and other localities possessing a coralline facies. "Thirdly: Finally above all these comes the Argilo-bituminous limestone with hydraulic cement, the fauna of which has been so carefully studied by M. Pictet. We cannot doubt then that this is the Lower Neocomien corresponding to the beds of Berrias.

"The impending discussion can therefore only bear upon the beds No. 2. Now it appears to me from the palæontological studies in connection with the fauna of this layer, that it has strong affinities with the Neocomien and contains no Ammonite of a decidedly Jurassic type. As to the Echinoderms and the Brachiopoda found in the breccia of Aizy, upon which my former opinion was chiefly grounded, they are open to discussion. Even supposing some incontestably Jurassic species should be found therein, as for example Acropeltis æquituberculata, Megerlea pectunculoides, Terebratulina substriata, etc., it is necessary to remark that they occur in a puddingstone, and in such a conglomerate, it is not impossible that these fossils, detached perhaps from the Jurassic beds, should have intermingled with the Neocomien ones, derived from the limestone immediately underneath. To the mind of any one acquainted with the locality, this breccia is evidently a shore-deposit, and this mixture of fossils, under such circumstances of deposition, would not be impossible. Therefore I am now entirely of the opinion of M. Hébert, or rather, I believe that the palæontological discussions raised on this subject will require to be solved in accordance with his opinion, which appears the only one that would agree with my stratigraphical studies.

"It is clear from the comparative table I have sent you, that on both sides, in the Jura and in the Alps there exists a break