Discourses: Biological and Geological — On Some of the Results of the Expedition of H.M.S. Challenger Pt 3

Professor Thomson gives the following account of this capital discovery:-- "According to our present experience, the deposit of _Globigerina_ ooze is limited to water of a certain depth, the extreme limit of the pure characteristic formation being placed at a depth of somewhere about 2,250 fathoms.

Crossing from these shallower regions occupied by the ooze into deeper soundings, we find, universally, that the calcareous formation gradually passes into, and is finally replaced by, an extremely fine pure clay, which occupies, speaking generally, all depths below 2,500 fathoms, and consists almost entirely of a silicate of the red oxide of iron and alumina.

The transition is very slow, and extends over several hundred fathoms of increasing depth; the shells gradually lose their sharpness of outline, and assume a kind of 'rotten' look and a brownish colour, and become more and more mixed with a fine amorphous red-brown powder, which increases steadily in proportion until the lime has almost entirely disappeared.

This brown matter is in the finest possible state of subdivision, so fine that when, after sifting it to separate any organisms it might contain, we put it into jars to settle, it remained for days in suspension, giving the water very much the appearance and colour of chocolate.

"In indicating the nature of the bottom on the charts, we came, from experience and without any theoretical considerations, to use three terms for soundings in deep water.

Two of these, Gl.

oz.

and r. cl.

, were very definite, and indicated strongly-marked formations, with apparently but few characters in common; but we frequently got soundings which we could not exactly call '_Globigerina_ ooze' or 'red clay,' and before we were fully aware of the nature of these, we were in the habit of indicating them as 'grey ooze' (gr.

oz.

) We now recognise the 'grey ooze' as an intermediate stage between the _Globigerina_ ooze and the red clay; we find that on one side, as it were, of an ideal line, the red clay contains more and more of the material of the calcareous ooze, while on the other, the ooze is mixed with an increasing proportion of 'red clay.'

"Although we have met with the same phenomenon so frequently, that we were at length able to predict the nature of the bottom from the depth of the soundings with absolute certainty for the Atlantic and the Southern Sea, we had, perhaps, the best opportunity of observing it in our first section across the Atlantic, between Teneriffe and St. Thomas.

The first four stations on this section, at depths from 1,525 to 2,220 fathoms, show _Globigerina_ ooze.

From the last of these, which is about 300 miles from Teneriffe, the depth gradually increases to 2,740 fathoms at 500, and 2,950 fathoms at 750 miles from Teneriffe.

The bottom in these two soundings might have been called 'grey ooze,' for although its nature has altered entirely from the _Globigerina_ ooze, the red clay into which it is rapidly passing still contains a considerable admixture of carbonate of lime.

"The depth goes on increasing to a distance of 1,150 miles from Teneriffe, when it reaches 3,150 fathoms; there the clay is pure and smooth, and contains scarcely a trace of lime.

From this great depth the bottom gradually rises, and, with decreasing depth, the grey colour and the calcareous composition of the ooze return.

Three soundings in 2,050, 1,900, and 1,950 fathoms on the 'Dolphin Rise' gave highly characteristic examples of the _Globigerina_ formation.

Passing from the middle plateau of the Atlantic into the western trough, with depths a little over 3,000 fathoms, the red clay returned in all its purity; and our last sounding, in 1,420 fathoms, before reaching Sombrero, restored the _Globigerina_ ooze with its peculiar associated fauna.

"This section shows also the wide extension and the vast geological importance of the red clay formation.

The total distance from Teneriffe to Sombrero is about 2,700 miles.

Proceeding from east to west, we have-- About 80 miles of volcanic mud and sand, " 350 " _Globigerina_ ooze, " 1,050 " red clay, " 330 " _Globigerina_ ooze, " 850 " red clay, " 40 " _Globigerina_ ooze;

giving a total of 1,900 miles of red clay to 720 miles of _Globigerina_ ooze.

"The nature and origin of this vast deposit of clay is a question of the very greatest interest; and although I think there can be no doubt that it is in the main solved, yet some matters of detail are still involved in difficulty.

My first impression was that it might be the most minutely divided material, the ultimate sediment produced by the disintegration of the land, by rivers and by the action of the sea on exposed coasts, and held in suspension and distributed by ocean currents, and only making itself manifest in places unoccupied by the _Globigerina_ ooze.

Several circumstances seemed, however, to negative this mode of origin.

The formation seemed too uniform: wherever we met with it, it had the same character, and it only varied in composition in containing less or more carbonate of lime.

"Again, the were gradually becoming more and more convinced that all the important elements of the _Globigerina_ ooze lived on the surface, and it seemed evident that, so long as the condition on the surface remained the same, no alteration of contour at the bottom could possibly prevent its accumulation; and the surface conditions in the Mid-Atlantic were very uniform, a moderate current of a very equal temperature passing continuously over elevations and depressions, and everywhere yielding to the tow-net the ooze-forming _Foraminifera_ in the same proportion.

The Mid-Atlantic swarms with pelagic _Mollusca_, and, in moderate depths, the shells of these are constantly mixed with the _Globigerina_ ooze, sometimes in number sufficient to make up a considerable portion of its bulk.

It is clear that these shells must fall in equal numbers upon the red clay, but scarcely a trace of one of them is ever brought up by the dredge on the red clay area.

It might be possible to explain the absence of shell-secreting animals living on the bottom, on the supposition that the nature of the deposit was injurious to them; but then the idea of a current sufficiently strong to sweep them away is negatived by the extreme fineness of the sediment which is being laid down; the absence of surface shells appears to be intelligible only on the supposition that they are in some way removed.

"We conclude, therefore, that the 'red clay' is not an additional substance introduced from without, and occupying certain depressed regions on account of some law regulating its deposition, but that it is produced by the removal, by some means or other, over these areas, of the carbonate of lime, which forms probably about 98 per cent.

of the material of the _Globigerina_ ooze.

We can trace, indeed, every successive stage in the removal of the carbonate of lime in descending the slope of the ridge or plateau where the _Globigerina_ ooze is forming, to the region of the clay.

We find, first, that the shells of pteropods and other surface _Mollusca_ which are constantly falling on the bottom, are absent, or, if a few remain, they are brittle and yellow, and evidently decaying rapidly.

These shells of _Mollusca_ decompose more easily and disappear sooner than the smaller, and apparently more delicate, shells of rhizopods.

The smaller _Foraminifera_ now give way, and are found in lessening proportion to the larger; the coccoliths first lose their thin outer border and then disappear; and the clubs of the rhabdoliths get worn out of shape, and are last seen, under a high power, as infinitely minute cylinders scattered over the field.

The larger _Foraminifera_ are attacked, and instead of being vividly white and delicately sculptured, they become brown and worn, and finally they break up, each according to its fashion; the chamber-walls of _Globigerina_ fall into wedge-shaped pieces, which quickly disappear, and a thick rough crust breaks away from the surface of _Orbulina_, leaving a thin inner sphere, at first beautifully transparent, but soon becoming opaque and crumbling away.

"In the meantime the proportion of the amorphous 'red clay' to the calcareous elements of all kinds increases, until the latter disappear, with the exception of a few scattered shells of the larger _Foraminifera_, which are still found even in the most characteristic samples of the 'red clay.'

"There seems to be no room left for doubt that the red clay is essentially the insoluble residue, the _ash_, as it were, of the calcareous organisms which form the _Globigerina_ ooze, after the calcareous matter has been by some means removed.

An ordinary mixture of calcareous _Foraminifera_ with the shells of pteropods, forming a fair sample of _Globigerina_ ooze from near St. Thomas, was carefully washed, and subjected by Mr. Buchanan to the action of weak acid; and he found that there remained after the carbonate of lime had been removed, about 1 per cent.

of a reddish mud, consisting of silica, alumina, and the red oxide of iron.

This experiment has been frequently repeated with different samples of _Globigerina_ ooze, and always with the result that a small proportion of a red sediment remains, which possesses all the characters of the red clay."

* * * * * "It seems evident from the observations here recorded, that _clay_, which we have hitherto looked upon as essentially the product of the disintegration of older rocks, may be, under certain circumstances, an organic formation like chalk; that, as a matter of fact, an area on the surface of the globe, which we have shown to be of vast extent, although we are still far from having ascertained its limits, is being covered by such a deposit at the present day.

"It is impossible to avoid associating such a formation with the fine, smooth, homogeneous clays and schists, poor in fossils, but showing worm- tubes and tracks, and bunches of doubtful branching things, such as Oldhamia, silicious sponges, and thin-shelled peculiar shrimps.

Such formations, more or less metamorphosed, are very familiar, especially to the student of palaeozoic geology, and they often attain a vast thickness.

One is inclined, from the great resemblance between them in composition and in the general character of the included fauna, to suspect that these may be organic formations, like the modern red clay of the Atlantic and Southern Sea, accumulations of the insoluble ashes of shelled creatures.

"The dredging in the red clay on the 13th of March was usually rich.

The bag contained examples, those with calcareous shells rather stunted, of most of the characteristic deep-water groups of the Southern Sea, including _Umbellularia, Euplectella, Pterocrinus, Brisinga, Ophioglypha, Pourtalesia_, and one or two _Mollusca_.

This is, however, very rarely the case.

Generally the red clay is barren, or contains only a very small number of forms."

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 Professor 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 _Diatomacea_, 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 two or three hundred fathoms of the surface--for anything 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 three thousand 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 Linnaeus enabled him to say that "fossils are not the children but the parents of rocks,"[9] 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 siliceous 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 Wyville Thomson and his colleagues--that the red clay is the residuum left after the calcareous matter of the _Globigerinoe_ 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.

[Footnote 9: "Petrificata montium calcariorum non filii sed parentes sunt, cum omnis calx oriatur ab animalibus."

--_Systema Naturae_, Ed.

xii.

, t. iii.

, p. 154. It must be recollected that Linnaeus 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, nec vice versa.

Sic runes saxei non primaevi, sed temporis filiae."

] So long as the _Globigerinoe_;, 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 "_vitae filiae_"; 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 pre-existing forms.

It is undeniable, for example, that the evidence in favour 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 palaeontology 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 palaeontological 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 co-extensive 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 towards the primitive form from 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 labours 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 favour of the same slow and gradual changes occurring then as now.

But this conclusion in nowise 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.