CHAPTER X.

THE CONTINUITY OF THE CHALK.

Points of Resemblance between the Atlantic Ooze and the White Chalk.—Differences between them.-Composition of Chalk.-The Doctrine of the Continuity of the Chalk.-Objections.-Arguments in favour of the View from Physical Geology and Geography.Former Distribution of Sea and Land.-Palæontological Evidence. --Chalk-flints.-Modern Sponges, and Ventriculites.- Corals. Echinoderms.-Mollusca.-Opinions of Professor Huxley and Mr. Prestwich. The Composition of Sea-water.-Presence of Organic Matter. Analysis of the contained Gases.-Differences of Specific Gravity. Conclusion.

APPENDIX A.-Summary of the Results of the Examination of Samples of Sea-water taken at the Surface and at various Depths. By William Lant Carpenter, B. A., B.Sc.

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APPENDIX B.-Results of the Analyses of Eight Samples of Sea-water collected during the Third Cruise of the Porcupine.' By Dr. Frankland, F.R.S.

APPENDIX C.-Notes on Specimens of the Bottom collected during the First Cruise of the 'Porcupine' in 1869. By David Forbes, F.R.S.

APPENDIX D.-Note on the Carbonic Acid contained in Sea-water. By John Young Buchanan, M.A., Chemist to the 'Challenger' Expedition.

VERY speedily after the first samples of the bottom of the mid-Atlantic had been brought up by the sounding-line, and submitted to chemical analysis and

to microscopical examination, many observers were struck with the great similarity between its composition and structure and that of the ancient chalk. I have already described the general character and the mode of origin of the great calcareous deposit which seems to occupy the greater part of the bed of the Atlantic. If we take a piece of the ordinary soft white chalk of the south of England, wash it down with a brush in water, and place a drop of the milky product on the slide of a microscope, we find that it consists, like the Atlantic ooze, of a large proportion of fine amorphous particles of lime, with here and there a portion of a Globigerina shell, and more rarely one of these shells entire, and a considerable proportion-in some examples coming up to nearly one-tenth of the whole of 'coccoliths,' which are indistinguishable from those of the ooze. Altogether two slides-one of washed down white chalk, and the other of Atlantic ooze-resemble one another so clearly, that it is not always easy for even an accomplished microscopist to distinguish them. The nature of chalk can also be well shown, as has been done by Ehrenberg and Sorby, by cutting it into thin diaphanous slices, when the mode of aggregation of the different materials can be readily demonstrated.

But while successive observers have brought out more and more clearly those resemblances,—sufficiently striking to place it beyond a doubt that the chalk of the cretaceous period and the chalk-mud of the modern Atlantic are substantially the same,-a more careful investigation shows that there are very important differences between them. The white chalk is very homogeneous, more so perhaps than any other

sedimentary rock, and may be said to be almost pure carbonate of lime. I quote an analysis of the white chalk of Shoreham (Sussex), by Mr. David Forbes.1

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Even the grey chalk of Folkestone contains a very large proportion of carbonate of lime, the other substances existing merely as impurities which can scarcely be said to enter into the composition of the rock. The following is an analysis by Mr. Forbes of the base of the Folkestone grey chalk chalk :

The most remarkable point in this analysis is that while white chalk is almost always associated with chert and flints, the chalk itself does not contain a particle of silica.

The chalk-mud of the Atlantic on the other hand contains not more than 60 per cent. of calcium carbonate, with 20 to 30 per cent. of silica, and varying proportions of alumina, magnesia, and oxide of iron. We must remember, however, that in the English

1 Quoted in Mr. Prestwich's Presidential Address, 1871.

cliffs we have the chalk in its very purest form, and that in various parts of the world it assumes a very different character, and contains carbonate of lime in very different proportions. Mr. Prestwich instances a bed 28 to 30 feet thick of the white chalk (Terrain Senonien) of Touraine, in which carbonate of lime is entirely absent.

There can be no doubt whatever that we have forming at the bottom of the present ocean, a vast sheet of rock which very closely resembles chalk; and there can be as little doubt that the old chalk, the cretaceous formation which in some parts of England has been subjected to enormous denudation, and which is overlaid by the beds of the tertiary series, was produced in the same manner, and under closely similar circumstances; and not the chalk only, but most probably all the great limestone formations. In almost all of these the remains of foraminifera are abundant, some of them apparently specifically identical with living forms; and in a large number of limestones of all ages Dr. Gümbel has detected the characteristic coccoliths.'

Long before commencing the present investigation, certain considerations had led me to regard it as highly probable that in the deeper parts of the Atlantic a deposit, differing possibly from time to time in composition but always of the same general character, might have been accumulating continuously from the cretaceous or even earlier periods to the present day. This view I suggested in my first letter to Dr. Carpenter urging the exploration of the seabed; and from the first it has had the cordial support of my colleague, whose intimate acquaintance with

some of the animal groups whose remains enter most largely into the chalk both old and new, makes his opinion on such a question particularly valuable.

On our return from the Lightning' cruise, during which we believed that our speculation had received strong confirmation, we used the expression,-perhaps somewhat an unfortunate one since it was capable of misconstruction,-that we might be regarded in a certain sense as still living in the cretaceous period. Several very eminent geologists, among whom were Sir Roderick Murchison and Sir Charles Lyell, took exception to this statement; but it seems that their censure was directed less against the opinion than the mode in which it was expressed; and I think I may say that the doctrine of the continuity of the chalk, in the sense in which we understood it, is now very generally accepted.

I do not maintain that the phrase we are still living in the cretaceous epoch,' is defensible in a strictly scientific sense, chiefly because the terms 'geological epoch' and 'geological period' are thoroughly indefinite. We speak indifferently of the Silurian period,' and the Glacial period,' without consideration of their totally unequal value; and of the Tertiary period,' and of the Miocene period,' although the one includes the other. The expression is intended rather in a popular sense to meet what was certainly until very lately the general popular impression, that a geological period has, in the region where it has been studied and defined, something like a beginning and an end; that it is bounded by periods of change-elevation, denudation, or some other evidence of the lapse of