Popular Science Monthly/Volume 7/May 1875/On Some of the Results of the Expedition of H M S Challenger
|ON SOME OF THE RESULTS OF THE EXPEDITION OF H. M. S. CHALLENGER.|
IN May, 1873, I drew attention, in the pages of this Review, to the important problems connected with the physics and natural history of the sea, to the solution of which there was every reason to hope the cruise of H. M. S. Challenger would furnish important contributions. The expectation then expressed has not been disappointed. Reports to the Admiralty, papers communicated to the Royal Society, and large collections which have already been sent home, have shown that the Challenger's staff have made admirable use of their great opportunities; and that, on the return of the expedition in 1874, their performance will be fully up to the level of their promise. Indeed, I am disposed to go so far as to say that, if nothing more came of the Challenger's expedition than has hitherto been yielded by her exploration of the nature of the sea-bottom at great depths, a full scientific equivalent of the trouble and expense of her equipment would have been obtained.
In order to justify this assertion, and yet, at the same time, not to claim more for Prof. Wyville Thomson and his colleagues than is their due, I must give a brief history of the observations which have preceded their exploration of this recondite field of research, and endeavor to make clear what was the state of knowledge in December, 1872, and what new facts have been added by the scientific staff of the Challenger. So far as I have been able to discover, the first successful attempt to bring up from great depths more of the sea-bottom than would adhere to a sounding-lead, was made by Sir John Ross, in the voyage to the arctic regions which he undertook in 1818. In the Appendix to the narrative of that voyage, there will be found an account of a very ingenious apparatus called "chlams"—a sort of double scoop—of his own contrivance, which Sir John Ross had made by the ship's armorer; and by which, being in Baffin's Bay, in 72° 30' north, and 77° 15' west, he succeeded in bringing up from 1,050 fathoms (or 6,300 feet), "several pounds" of a "fine green mud," which formed the bottom of the sea in this region. Captain (now Sir Edward) Sabine, who accompanied Sir John Ross on this cruise, says of this mud that it was "soft and greenish, and that the lead sunk several feet into it." A similar "fine green mud" was found to compose the sea-bottom in Davis Straits by Goodsir in 1845. Nothing is certainly known of the exact nature of the mud thus obtained, but we shall see that the mud of the bottom of the antarctic seas is described in curiously similar terms by Dr. Hooker, and there is no doubt as to the composition of this deposit.
In 1850 Captain Penny collected in Assistance Bay, in Kingston Bay, and in Melville Bay, which lie between 73° 45' and 74° 40' north, specimens of the residuum left by melted surface-ice, and of the sea-bottom in these localities. Dr. Dickie, of Aberdeen, sent these materials to Ehrenberg, who made out that the residuum of the melted ice consisted for the most part of the silicious cases of diatomaceous plants, and of the silicious spicula of sponges; while, mixed with these, were a certain number of the equally silicious skeletons of those low animal organisms, which were termed Polycistinæe of Ehrenberg, but are now known as Radiolaria.
In 1856 a very remarkable addition to our knowledge of the nature of the sea-bottom in high northern latitudes was made by Prof. Bailey of West Point. Lieutenant Brooke, of the United States Navy, who was employed in surveying the Sea of Kamtchatka, had succeeded in obtaining specimens of the sea-bottom from greater depths than any hitherto reached, namely, from 2,700 fathoms (16,200 feet) in 66° 46' north, and 168° 18' east; and from 1,700 fathoms (10,200 feet) in 60° 15' north, and 170° 53' east. On examining these microscopically, Prof. Bailey found, as Ehrenberg had done in the case of mud obtained on the opposite side of the arctic region, that the fine mud was made up of shells of Diatomaceæ, of spicula of sponges, and of Radiolaria, with a small admixture of mineral matters, but without a trace of any calcareous organisms.
Still more complete information has been obtained concerning the nature of the sea-bottom in the cold zone around the south pole. Between the years 1839 and 1843, Sir James Clark Ross executed his famous antarctic expedition, in the course of which he penetrated, at two widely-distant points of the antarctic zone, into the high latitudes of the shores of Victoria Land and of Graham's Land, and reached the parallel of 80° south. Sir James Eoss was himself a naturalist of no mean acquirements, and Dr. Hooker, the present President of the Royal Society, accompanied him as naturalist to the expedition, so that the observations upon the fauna and flora of the antarctic regions made during this cruise were sure to have a peculiar value and importance, even had not the attention of the voyagers been particularly directed to the importance of noting the occurrence of the minutest forms of animal and vegetable life in the ocean.
Among the scientific instructions for the voyage drawn up by a committee of the Royal Society, however, there is a remarkable letter from Von Humboldt to Lord Minto, then First Lord of the Admiralty, in which, among other things, he dwells upon the significance of the researches into the microscopic composition of rocks, and the discovery of the great share which microscopic organisms take in the formation of the crust of the earth at the present day, made by Ehrenberg in the years 1836-'39. Ehrenberg, in fact, had shown that the extensive beds of "rotten-stone" or "Tripoli" which occur in various parts of the world, and notably at Bilin in Bohemia, consisted of accumulations of the silicious cases and skeletons, Diatomaceæ, sponges, and Radiolaria; he had proved that similar deposits were being formed by Diatomaceæ in the pools of the Thiergarten, in Berlin and elsewhere, and had pointed out that, if it were commercially worth while, rotten-stone might be manufactured by a process of diatom-culture. Observations, conducted at in 1839, had revealed the existence, at the surface of the waters of the Baltic, of living diatoms and Radiolaria of the same species as those which, in a fossil state, constitute extensive rocks of Tertiary age at Caltanisetta, Zante, and Oran, on the shores of the Mediterranean.
Moreover, in the fresh-water rotten-stone beds of Bilin, Ehrenberg had traced out the metamorphosis, effected apparently by the action of percolating water, of the primitively loose and friable deposit of organized particles, in which the silex exists in the hydrated or soluble condition. The silex, in fact, undergoes solution and slow redeposition, until, in ultimate result, the excessively tine-grained sand, each particle of which is a skeleton, becomes converted into a dense opaline stone, with only here and there an indication of an organism.
From the consideration of these facts, Ehrenberg, as early as the year 1839, had arrived at the conclusion that rocks, altogether similar to those which constitute a large part of the crust of the earth, must be forming, at the present day, at the bottom of the sea; and he threw out the suggestion that even where no traces of organic structure is to be found in the older rocks, it may have been lost by metamorphosis.
The results of the antarctic exploration, as stated by Dr. Hooker in the "Botany of the Antarctic Voyage," and in a paper which he read before the British Association in 1847, are of the greatest importance in connection with these views, and they are so clearly stated in the former work, which is somewhat inaccessible, that I make no apology for quoting them at length:
With respect to the distribution of the Diatomaceæ, Dr. Hooker remarks:
The Challenger has explored the antarctic seas in a region intermediate between those examined by Sir James Ross's expedition; and the observations made by Dr. Wyville Thomson and his colleagues in every respect confirm those of Dr. Hooker:
The observations which have been detailed leave no doubt that the antarctic sea-bottom, from a little to the south of the fiftieth parallel, as far as 80 south, is being covered by a fine deposit of silicious mud, more or less mixed, in some parts, with the ice-borne débris of polar lands and with the ejections of volcanoes. The silicious particles which constitute this mud are derived, in part, from the diatomaceous plants and radiolarian animals which throng the surface, and, in part, from the spicula of sponges which live at the bottom. The evidence respecting the corresponding arctic area is less complete, but it is sufficient to justify the conclusion that an essentially similar silicious cap is being formed around the northern pole.
There is no doubt that the constituent particles of this mud may agglomerate into a dense rock, such as that formed at Oran, on the shores of the Mediterranean, which is made up of similar materials. Moreover, in the case of fresh-water deposits of this kind, it is certain that the action of percolating water may convert the originally soft and friable, fine-grained sandstone into a dense semi-transparent opaline stone, the silicious organized skeletons' being dissolved, and the silex redeposited in an amorphous state. Whether such a metamorphosis as this occurs in submarine deposits, as well as in those formed in fresh water, does not appear; but there seems no reason to doubt that it may. And hence it may not be hazardous to conclude that very ordinary metamorphic agencies may convert these polar caps into a form of quartzite.
In the great intermediate zone, occupying some 110 of latitude, which separates the circurapolar arctic and antarctic areas of silicious deposit, the diatoms and Radiolaria of the surface-water and the sponges of the bottom do not die out, and, so far as some forms are concerned, do not even appear to diminish in total number; though, on a rough estimate, it would appear that the proportion of Radiolaria to diatoms is much greater than in the colder seas. Nevertheless the composition of the deep-sea mud of this intermediate zone is entirely different from that of the circumpolar regions.
The first exact information respecting the nature of this mud at depths greater than 1,000 fathoms was given by Ehrenberg, in the account which he published in the "Monatsberichte" of the Berlin Academy for the year 1853, of the soundings obtained by Lieutenant Berryman, of the United States Navy, in the North Atlantic, between Newfoundland and the Azores.
Observations which confirm those of Ehrenberg in all essential respects have been made by Prof. Bailey, myself. Dr. Wallich, Dr. Carpenter, and Prof. Wyville Thomson, in their earlier cruises; and the continuation of the Globigerina ooze over the South Pacific has been proved by the recent work of the Challenger, by which it is also shown, for the first time, that, in passing from the equator to high southern latitudes, the number and variety of the Foramimfera diminish, and even the Glbhigerinæ become dwarfed. And this result, it will be observed, is in entire accordance with the fact already mentioned that, in the sea of Kamtchatka, the deep sea mud was found by Bailey to contain no calcareous organisms.
Thus, in the whole of the "intermediate zone," the silicious deposit which is being formed there, as elsewhere, by the accumulation of sponge-spicula, Radiolaria, and diatoms, is obscured and overpowered by the immensely greater amount of calcareous sediment, which arises from the aggregation of the skeletons of dead Foraminifera. The similarity of the deposit, thus composed of a large percentage of carbonate of lime, and a small percentage of silex, to chalk, regarded merely as a kind of rock, which was first pointed out by Ehrenberg, is now admitted on all hands; nor can it be reasonably doubted that ordinary metamorphic agencies are competent to convert the "modern chalk" into hard limestone, or even into crystalline marble.
Ehrenberg appears to have taken it for granted that the Globigerinæ and other Foraminifera which are found in the deep-sea mud, live at the great depths in which their remains are found; and he supports this opinion by producing evidence that the soft parts of these organisms are preserved, and may be demonstrated by removing the calcareous matter with dilute acids. In 1857 the evidence for and against this conclusion appeared to me to be insufficient to warrant a positive conclusion one way or the other, and I expressed myself, in my report to the Admiralty on Captain Dayman's soundings, in the following terms:
"Three answers have suggested themselves:
"In accordance with the prevalent view of the limitation of life to comparatively small depths, it is imagined either: 1. That these organisms have drifted into their present position from shallower waters; or 2. That they habitually live at the surface of the ocean, and only fall down into their present position.
"1. I conceive that the first supposition is negatived by the extremely marked zoological peculiarity of the deep-sea fauna.
"Had the Globigerinæ been drifted into their present position from shallow water, we should find a very large proportion of the characteristic inhabitants of shallow water mixed with them, and this would the more certainly be the case, as the large Globgerinæ so abundant in the deep-sea soundings, are, in proportion to their size, more massive and solid than almost any other Foraminifera. But the fact is, that the proportion of other Foramimfera is exceedingly small, nor have I found as yet, in the deep-sea deposits, any such matters as fragments of molluscous shells, of Echini etc., which abound in shallow waters, and are quite as likely to be drifted as the heavy Globigerinæ. Again, the relative proportions of young and fully-formed Globigerhice seem inconsistent with the notion that they have traveled far. And it seems difficult to imagine why, had the deposit been accumulated in this way, Coscinodisci should so almost entirely represent the Diatomaceæ.
"2. The second hypothesis is far more feasible, and is strongly supported by the fact that many Polycistineæ (Radiolaria) and Coscinodisci are well known to live at the surface of the ocean. Mr. Macdonald, Assistant Surgeon of H. M. S. Herald, now in the Southwestern Pacific, has lately sent home some very valuable observations on living forms of this kind, met with in the stomachs of oceanic mollusks, and therefore certainly inhabitants of the superficial layer of the ocean. But it is a singular circumstance that only one of the forms figured by Mr. Macdonald is at all like a Globigerina, and there are some peculiarities about even this which make me greatly doubt its affinity with that genus. The form, indeed, is not unlike that of a Globigerina, but it is provided with long radiating processes, of which I have never seen any trace in Globigerina. Did they exist, they might explain what otherwise is a great objection to this view, viz., how is it conceivable that the heavy Globgerina should maintain itself at the surface of the water?
"If the organic bodies in the deep-sea soundings have neither been drifted, nor have fallen from above, there remains but one alternative—they must have lived and died as they are.
"Important objections, however, at once suggest themselves to this view. How can animal life be conceived to exist under such conditions of light, temperature, pressure, and aeration as must obtain at these vast depths?
"To this one can only reply that we know for a certainty that even very highly-organized animals do continue to live at a depth of 300 and 400 fathoms, inasmuch as they have been dredged up thence; and that the difference in the amount of light and heat at 400 and 2,000 fathoms is probably, so to speak, very far less than the difference in complexity of organization between these animals and the humbler Protozoa and Protophyta of the deep-sea soundings.
"I confess, though as yet far from regarding it proved that the Globigerinæ live at these depths, the balance of probabilities seems to me to incline in that direction. And there is one circumstance which weighs strongly in my mind. It may be taken as a law that any genus of animals that is found far back in time is capable of living under a great variety of circumstances as regards light, temperature, and pressure. Now, the genus Globigerina is abundantly represented in the Cretaceous epoch, and perhaps earlier."I abstain, however, at present from drawing any positive conclusions, preferring rather to await the result of more extended observations."
Dr. Wallich, Prof. Wyville Thomson, and Dr. Carpenter, concluded that the Globigerinæ live at the bottom. Dr. Wallich writes in 1862: "By sinking very fine gauze-nets to considerable depths, I have repeatedly satisfied myself that Globigerina does not occur in the superficial strata of the ocean." Moreover, having obtained certain living star-fish from a depth of 1,260 fathoms, and found their stomachs full of "fresh-looking Globigerinæ" and their débris he adduces this fact in support of his belief that the Globigerinæ live at the bottom.
On the other hand, Müller, Häckel, Major Owen, Mr. Gwyn Jeffries, and other observers, found that Globigerinæ, with. the allied genera Orbulina and Pulvinulina. sometimes occur abundantly at the surface of the sea, the shells of these pelagic forms being not unfrequently provided with the long spines noticed by Macdonald; and in 1865 and 1866 Major Owen more especially insisted on the importance of this fact. The recent work of the Challenger fully confirms Major Owen's statement. In the paper recently published in the proceedings of the Royal Society, from which a quotation has already been made. Prof. Wyville Thomson says:
"Mr. Murray has combined with a careful examination of the soundings a constant use of the tow-net, usually at the surface, but also at depths of from 10 to 100 fathoms, and he finds the closest relation to exist between the surface fauna of any particular locality and the deposit which is taking place at the bottom. In all seas, from the equator to the polar ice, the tow-net contains Globigerinæ. They are more abundant and of a larger size in warmer seas; several varieties, attaining a large size, and presenting marked varietal characters, are found in the intertropical area of the Atlantic. In the latitude of Kerguelen they are less numerous and smaller, while farther south they are still more dwarfed, and only one variety, the typical Globigerina bulloides, is represented. The living Globigerinæ from the tow-net are singularly different in appearance from the dead shells we find at the bottom. The shell is clear and transparent, and each of the pores which penetrate it is surrounded by a raised crest, the crest round adjacent pores coalescing into a roughly hexagonal network, so that the pores appear to lie at the bottom of an hexagonal pit. At each angle of this hexagon the crest gives off a delicate, flexible, calcareous spine, which is sometimes four or five times the diameter of the shell in length. The spines radiate symmetrically from the direction of the centre of each chamber of the shell, and the sheaves of long transparent needles crossing one another in different directions have a very beautiful effect. The smaller inner chambers of the shell are entirely filled with an orange-yellow granular sarcode; and the large terminal chamber usually contains only a small irregular mass, or two or three small masses run together, of the same yellow sarcode stuck against one side, the remainder of the chamber being empty. No definite arrangement and no approach to structure was observed in the sarcode, and no differentiation, with the exception of round bright-yellow oil-globules, very much like those found in some of the radiolarians which are scattered, apparently irregularly, in the sarcode. We never have been able to detect, in any of the large number of Globigerinæ which we have examined, the least trace of pseudopodia, or any extension, in any form, of the sarcode beyond the shell. . . ."In specimens taken with the tow-net the spines are very usually absent; but that is probably on account of their extreme tenuity; they are broken off by the slightest touch. In fresh examples from the surface, the dots indicating the origin of the lost spines may almost always be made out with a high power. There are never spines on the Globigerinæ from the bottom, even in the shallowest water."
There can now be no doubt, therefore, that the Globigerinæ live at the top of the sea; but the question may still be raised whether they do not also live at the bottom. In favor of this view, it has been urged that the shells of the Globigerinæ of the surface never possess such thick walls as those which are found at the bottom, but I confess that I doubt the accuracy of this statement. Again, the occurrence of minute Globigerinæ in all stages of development, at the greatest depths, is brought forward as evidence that they live in situ. But, considering the extent to which the surface-organisms are devoured, without discrimination of young and old, by Salpæ and the like, it is not wonderful that the shells of all ages should be among the rejectamenta. Nor can the presence of the soft parts of the body in the shells which form the Globigerina ooze, and the fact, if it be one, that animals living at the bottom use them as food, be considered as conclusive evidence that the Globigerinæ live at the bottom. Such as die at the surface, and even many of those which are swallowed up by other animals, may retain much of their protoplasmic matter when they reach the depths at which the temperature sinks to 34° or 32° Fahr., where decomposition must become exceedingly slow.
Another consideration appears to me to be in favor of the view that the Globigerinæ and their allies are essentially surface-animals. This is the fact brought out by the Challenger's work, that they have a southern limit of distribution, which can hardly depend upon any thing but the temperature of the surface-water. And it is to be remarked that this southern limit occurs at a lower latitude in the antarctic seas than it does in the North Atlantic. According to Dr. Wallich ("The North-Atlantic Sea-Bed," p. 157) Globigerina is the prevailing form in the deposits between the Farœ Islands and Iceland, and between Iceland and East Greenland—or, in other words, in a region of the sea-bottom which lies altogether north of the parallel of 60° north; while in the southern seas the Globigerinæ become dwarfed and almost disappear between 50° and 55° south. On the other hand, in the sea of Kamtchatka, the Globigerinæ have vanished in 56° north, so that the persistence of the Globigerina ooze in high latitudes, in the North Atlantic, would seem to depend on the northward curve of the isothermals peculiar to this region; and it is difficult to understand how the formation of Globigerina ooze can be affected by this climatal peculiarity unless it be effected by surface animals.
Whatever may be the mode of life of the Foraminifera, to which the calcareous element of the deep-sea "chalk" owes its existence, the fact that it is the chief and most widely-spread material of the sea-bottom in the intermediate zone, throughout both the Atlantic and Pacific Oceans, and the Indian Ocean, at depths from a few hundred to over 2,000 fathoms, is established. But it is not the only extensive deposit which is now taking place. In 1853 Count Pourtalès, an officer of the United States Coast Survey, which has done so much for scientific hydrography, observed that the mud forming the sea-bottom at depths of 150 fathoms, in 31° 32' north, and 79° 35' west, off the coast of Florida, was "a mixture, in about equal proportions, of Globigerinæ and black sand, probably green sand, as it makes a green mark when crushed on paper." Prof. Bailey, examining these grains microscopically, found that they were casts of the interior cavities of Foraminifera, consisting of a mineral known as Glauconite which is a silicate of iron and alumina. In these casts the minutest cavities and finest tubes in the Foraminifera were sometimes reproduced in solid counterparts of the glassy mineral, while the calcareous original had been entirely dissolved away.
Contemporaneously with these observations, the indefatigable Ehrenberg had discovered that the "greensands" of the geologist were largely made up of casts of a similar character, and proved the existence of Foraminifera at a very ancient geological epoch, by discovering such casts in a greensand of Lower Silurian age, which occurs near St. Petersburg.
Subsequently Messrs. Parker and Jones discovered similar casts in process of formation, the original shell not having disappeared, in specimens of the sea-bottom of the Australian seas, brought home by the late Prof. Jukes. And the Challenger has observed a deposit of a similar character in the course of the Agulhas current, near the Cape of Good Hope, and in some other localities not yet defined.
It would appear that this infiltration of Foraminifera shells with Glauconite does not take place at great depths, but rather in what may be termed a sublittoral region, ranging from 100 to 300 fathoms. It cannot be ascribed to any local cause, for it takes place, not only over large areas in the Gulf of Mexico and the coast of Florida, but in the South Atlantic and in the Pacific. But what are the conditions which determine its occurrence, and whence the silex, the iron, and the alumina (with perhaps potash and some other ingredients in small quantity) of which the Glauconite is composed, proceed, are points on which no light has yet been thrown. For the present we must be content with the fact that, in certain areas of the "intermediate zone," greensand is replacing and representing the primitive calcareo-silicious ooze.
The investigation of the deposits which are now being formed in the basin of the Mediterranean by the late Prof. Edward Forbes, by Prof. Williamson, and more recently by Dr. Carpenter, and a comparison of the results thus obtained with what is known of the surface fauna, have brought to light the remarkable fact that, while the surface and the shallows abound with Foraminifera and other calcareous-shelled organisms, the indications of life become scanty at depths beyond 500 or 600 fathoms, while almost all traces of it disappear at greater depths, and at 1,000 to 2,000 fathoms the bottom is covered with a fine clay.
Dr. Carpenter has discussed the significance of this remarkable fact, and he is disposed to attribute the absence of life at great depths partly to the absence of any circulation of the water of the Mediterranean at such depths, and partly to the exhaustion of the oxygen of the water by the organic matter contained in the fine clay, which he conceives to be formed by the finest particles of the mud brought down by the rivers which flow into the Mediterranean.
However this may be, the explanation thus offered of the presence of the fine mud, and of the absence of organisms which ordinarily live at. the bottom, does not account for the absence of the skeletons of the organisms which undoubtedly abound at the surface of the Mediterranean; and it would seem to have no application to the remarkable fact discovered by the Challenger, that in the open Atlantic and Pacific Oceans, in the midst of the great intermediate zone, and thousands of miles away from the embouchure of any river, the sea-bottom, at depths approaching to and beyond 3,000 fathoms, no longer consists of Globigerina ooze, but an excessively fine red clay.
Prof. Thomson gives the following account of this capital discovery:
|About||80||miles||of volcanic mud and sand,|
It must be admitted that it is very difficult at present to frame any satisfactory explanation of the mode of origin of this singular deposit of red clay.
I cannot say that the theory put forward tentatively, and with much reservation by Prof Thomson, that the calcareous matter is dissolved out by the relatively fresh water of the deep currents from the antarctic regions, appears satisfactory to me. Nor do I see my way to the acceptance of the suggestion of Dr. Carpenter, that the red clay is the result of the decomposition of previously-formed greensand. At present there is no evidence that greensand casts are ever formed at great depths; nor has it been proved that Glauconite is decomposable by the agency of water and carbonic acid.
I think it probable that we shall have to wait some time for a sufficient explanation of the origin of the abyssal red clay, no less than for that of the sublittoral greensand in the intermediate zone. But the importance of the establishment of the fact that these various deposits are being formed in the ocean, at the present day, remains the same, whether its rationale be understood or not.
For suppose the globe to be evenly covered with sea, to a depth say of a thousand fathoms—then, whatever might be the mineral matter composing the sea-bottom, little or no deposit would be formed upon it, the abrading and denuding action of water, at such a depth, being exceedingly slight. Next, imagine sponges, Radiolaria Foraminifera, and diatomaceous plants, such as those which now exist in the deep sea, to be introduced: they would be distributed according to the same laws as at present, the sponges (and possibly some of the Foraminifera) covering the bottom, while other Foraminifera, with the Radiolaria and Diatomaceæ would increase and multiply in the surface-waters. In accordance with the existing state of things, the Radiolaria and diatoms would have a universal distribution, the latter gathering most thickly in the polar regions, while the Foraminifera would be largely, if not exclusively, confined to the intermediate zone; and, as a consequence of this distribution, a bed of "chalk" would begin to form in the intermediate zone, while caps of silicious rock would accumulate on the circumpolar regions.
Suppose, further, that a part of the intermediate area were raised to within 200 or 300 fathoms of the surface—for any thing that we know to the contrary, the change of level might determine the substitution of greensand for the "chalk;" while, on the other hand, if part of the same area were depressed to 3,000 fathoms, that change might determine the substitution of a different silicate of alumina and iron—namely, clay—for the "chalk" that would otherwise be formed.
If the Challenger hypothesis, that the red clay is the residue left by dissolved Foraminiferous skeletons, is correct, then all these deposits alike would be directly, or indirectly, the product of living organisms. But just as a silicious deposit may be metamorphosed into opal or quartzite, and chalk into marble, so known metamorphic agencies may metamorphose clay into schist, clay-slate, slate, gneiss, or even granite. And thus, by the agency of the lowest and simplest of organisms, our imaginary globe might be covered with strata, of all the chief kinds of rock of which the known crust of the earth is composed, of indefinite thickness and extent.
The bearing of the conclusions which are now either established, or highly probable, respecting the origin of silicious, calcareous, and clayey rocks, and their metamorphic derivatives, upon the archaeology of the earth, the elucidation of which is the ultimate object of the geologist, is of no small importance.
A hundred years ago the singular insight of Linnæus enabled him to say that "fossils are not the children but the parents of rocks," and the whole effect of the discoveries made since his time has been to compile a larger and larger commentary upon this text. It is, at present, a perfectly tenable hypothesis that all silicious and calcareous rocks are either directly, or indirectly, derived from material which has, at one time or other, formed part of the organized framework of living organisms. Whether the same generalization may be extended to aluminous rocks, depends upon the conclusion to be drawn from the facts respecting the red-clay areas brought to light by the Challenger. If we accept the view taken by Mr. Wyville Thomson and his colleagues—that the red clay is the residuum left after the calcareous matter of the Globigerinæ ooze has been dissolved away—then clay is as much a product of life as limestone, and all known derivatives of clay may have formed part of animal bodies.
So long as the Globigerinæ, actually collected at the surface, have not been demonstrated to contain the elements of clay, the Challenger hypothesis, as I may term it, must be accepted with reserve and provisionally, but, at present, I cannot but think that it is more probable than any other suggestion which has been made.
Accepting it provisionally, we arrive at the remarkable result that all the chief known constituents of the crust of the earth may have formed part of living bodies; that they may be the "ash" of protoplasm; that the "rupes saxei" are not only "temporis," but "vitæ filiæ;" and, consequently, that the time during which life has been active on the globe may be indefinitely greater than the period the commencement of which is marked by the oldest known rocks, whether fossiliferous or unfossiliferous.
And thus we are led to see where the solution of a great problem and apparent paradox of geology may lie. Satisfactory evidence now exists that some animals in the existing world have been derived by a process of gradual modification from preëxisting forms. It is undeniable, for example, that the evidence in favor of the derivation of the horse from the later tertiary Hipparion, and that of the Hipparion from Anchitherium, is as complete and cogent as such evidence can reasonably be expected to be; and, the further investigations into the history of the tertiary mammalia are pushed, the greater is the accumulation of evidence having the same tendency. So far from paleontology lending no support to the doctrine of evolution—as one sees constantly asserted—that doctrine, if it had no other support, would have been irresistibly forced upon us by the paleontological discoveries of the last twenty years.
If, however, the diverse forms of life which now exist have been produced by the modification of previously-existing less divergent forms, the recent and extinct species, taken as a whole, must fall into series which must converge as we go back in time. Hence, if the period represented by the rocks is greater than, or coextensive with, that during which life has existed, we ought, somewhere among the ancient formations, to arrive at the point to which all these series converge, or from which, in other words, they have diverged—the primitive undifferentiated protoplasmic living things, whence the two great series of plants and animals have taken their departure.
But, as a matter of fact, the amount of convergence of series, in relation to the time occupied by the deposition of geological formations, is extraordinarily small. Of all animals the higher Vertebrata are the most complex; and among these the carnivores and hoofed animals (Ungulata) are highly differentiated. Nevertheless, although the different lines of modification of the Carnivora and those of the Ungulata, respectively, approach one another, and, although each group is represented by less differentiated forms in the older tertiary rocks than at the present day, the oldest tertiary rocks do not bring us near the primitive form of either. If, in the same way, the convergence of the varied forms of reptiles is measured against the time during which their remains are preserved—which is represented by the whole of the tertiary and mesozoic formations—the amount of that convergence is far smaller than that of the lines of mammals, between the present time and the beginning of the tertiary epoch. And it is a broad fact that, the lower we go in the scale of organization, the fewer signs are there of convergence toward the primitive form whence all must have diverged, if evolution be a fact. Nevertheless, that it is a fact in some cases, is proved, and I, for one, have not the courage to suppose that the mode in which some species have taken their origin is different from that in which the rest have originated.
What, then, has become of all the marine animals which, on the hypothesis of evolution, must have existed in myriads in those seas, wherein the many thousand feet of Cambrian and Laurentian rocks, now devoid, or almost devoid, of any trace of life, were deposited?
Sir Charles Lyell long ago suggested that the azoic character of these ancient formations might be due to the fact that they had undergone extensive metamorphosis; and readers of the "Principles of Geology" will be familiar with the ingenious manner in which he contrasts the theory of the Gnome, who is acquainted only with the interior of the earth, with those of ordinary philosophers, who know only its exterior.
The metamorphism contemplated by the great modern champion of rational geology is, mainly, that brought about by the exposure of rocks to subterranean heat, and, where no such heat could be shown to have operated, his opponents assumed that no metamorphosis could have taken place. But the formation of greensand, and still more that of the "red clay" (if the Challenger hypothesis be correct) affords an insight into a new kind of metamorphosis—not igneous, but aqueous—by which the primitive nature of a deposit may be masked as completely as it can be by the agency of heat. And, as Wyville Thomson suggests, in the passage I have quoted above (p. 17), it further enables us to assign a new cause for the occurrence, so puzzling hitherto, of thousands of feet of unfossiliferous fine-grained schists and slates, in the midst of formations deposited in seas which certainly abounded in life. If the great deposit of "red clay" now forming in the eastern valley of the Atlantic were metamorphosed into slate and then upheaved, it would constitute an "azoic" rock of enormous extent. And yet that rock is now forming in the midst of a sea which swarms with living beings, the great majority of which are provided with calcareous or silicious shells and skeletons, and therefore are such as, up to this time, we should have termed eminently preservable.
Thus the discoveries made by the Challenger expedition, like all recent advances in our knowledge of the phenomena of biology, or of the changes now being effected in the structure of the surface of the earth, are in accordance with, and lend strong support to, that doctrine of Uniformitarianism, which, fifty years ago, was held only by a small minority of English geologists—Lyell, Scrope, and De la Beche—but now, thanks to the long-continued labors of the first two, and mainly to those of Sir Charles Lyell, has gradually passed from the position of a heresy to that of catholic doctrine.
Applied within the limits of the time registered by the known fraction of the crust of the earth, I believe that Uniformitarianism is unassailable. The evidence that, in the enormous lapse of time between the deposition of the lowest Laurentian strata and the present day, the forces which have modified the surface of the crust of the earth were different in kind, or greater in the intensity of their action, than those which are now occupied in the same work, has yet to be produced. Such evidence as we possess all tends in the contrary direction, and is in favor of the same slow and gradual changes occurring then as now.
But this conclusion in no wise conflicts with the deductions of the physicist from his no less clear and certain data. It may be certain that this globe has cooled down from a condition in which life could not have existed; it may be certain that, in so cooling, its contracting crust must have undergone sudden convulsions, which were to our earthquakes as an earthquake is to the vibration caused by the periodical eruption of a geyser; but in that case the earth must, like other respectable parents, have sowed her wild-oats, and got through her turbulent youth, before we, her children, have any knowledge of her.
So far as the evidence afforded by the superficial crust of the earth goes, the modern geologist can, ex animo, repeat the saying of Hutton, "We find no vestige of a beginning—no prospect of an end." However, he will add, with Hutton, "But in thus tracing back the natural operations which have succeeded each other, and mark to us the course of time past, we come to a period in which we cannot see any further." And if he seek to peer into the darkness of this period, he will welcome the light proffered by physics and mathematics.—Contemporary Review.
- "Ueber neue Auschauungen des kleinsten nördlichen Polarlebens," Monatsberichte der Königlichen Akademie, Berlin, 1853.
- "Ueber die noch jetzt zahlreich lebenden Thierarten der Kreidebildung und den Organismus der Polythalamien," Abhandlungen der Königlichen Akademie der Wissenschaften, 1839. Berlin, 1841. I am afraid that this remarkable paper has been somewhat overlooked in the recent discussions of the relation of ancient rocks to modern deposits.
- The following passages, in Ehrenberg's memoir on "The Organisms in the Chalk which are still living" (1839), are conclusive: "7. The dawning period of the existing living organic creation, if such a period is distinguishable (which is doubtful), can only be supposed to have existed on the other side of, and below, the chalk formation; and thus, either the chalk, with its wide-spread and thick beds, must enter into the series of newer formations, or some of the accepted four great geological periods—the quaternary, tertiary, and secondary formations—contain organisms which still live. It is more probable, in the proportion of three to one, that the transition or primary period is not different, but that it is only more difficult to examine and understand, by reason of the gradual and prolonged chemical decomposition and metamorphosis of many of its organic constituents." "10. By the mass-forming Infusoria and Polythalamia, secondary are not distinguishable from tertiary formations; and, from what has been said, it is possible that, at this very day, rock-masses are forming in the sea, and being raised by volcanic agencies, the constitution of which, on the whole, is altogether similar to that of the chalk. The chalk remains distinguishable by its organic remains as a formation, but not as a kind of rock."
- Appendix to "Report on Deep-Sea Soundings in the Atlantic Ocean," by Lieutenant Commander Joseph Dayman, 1857.
- The "North Atlantic Sea-Bed," p. 137.
- "Preliminary Notes on the Nature of the Sea-Bottom procured by the Soundings of H. M. S. Challenger during her Cruise in the Southern Seas in the Early Part of the Year 1874."—(Proceedings of the Royal Society, November 26, 1874.)
- "Petrificata montium calcariorum non filii sed parentes sunt, cum omnis calx oriatur ab animalibus." "Systema Naturæ," Ed. xii., t. iii., p. 154. It must be recollected that Linnæus included silex, as well as limestone, under the name of "calx," and that he would probably have arranged diatoms among animals, as part of "chaos." Ehrenberg quotes another even more pithy passage, which I have not been able to find in any edition of the "Systema" accessible to me: "Sic lapides ab animalibus, nee vice versa. Sic rupes saxei non primævi, sed temporis filiæ."