Discourses: Biological and Geological — On Some of the Results of the Expedition of H.M.S. Challenger Pt 2

In the great intermediate zone, occupying some 110° of latitude, which separates the circumpolar 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 Lieut.

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 Professor Bailey, myself, Dr. Wallich, Dr. Carpenter, and Professor 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 _Foraminifera_ diminishes, and even the _Globigerinoe_ become dwarfed.

And this result, it will be observed, is in entire accordance with the fact already mentioned that, in the sea of Kamschatka, 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,[5] 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.

[Footnote 5: 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 widespread 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 3 to 1, 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 _Infasoria_ 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."

] Ehrenberg appears to have taken it for granted that the _Globigerinoe_ 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:-- "When we consider the immense area over which this deposit is spread, the depth at which its formation is going on, and its similarity to chalk, and still more to such rocks as the marls of Caltanisetta, the question, whence are all these organisms derived?

becomes one of high scientific interest.

"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 _Globigerinoe_ been drifted into their present position from shallow water, we should find a very large proportion of the characteristic inhabitants of shallow waters mixed with them, and this would the more certainly be the case, as the large _Globigerinoe_, so abundant in the deep-sea soundings, are, in proportion to their size, more solid and massive than almost any other _Foraminifera_.

But the fact is that the proportion of other _Foraminifera_ is exceedingly small, nor have I found as yet, in the deep-sea deposits, any such matters as fragments of molluscous shells, of _Echini_, &c. , which abound in shallow waters, and are quite as likely to be drifted as the heavy _Globigerinoe_.

Again, the relative proportions of young and fully formed _Globigerinoe_ seem inconsistent with the notion that they have travelled far.

And it seems difficult to imagine why, had the deposit been accumulated in this way, _Coscinodisci_ should so almost entirely represent the _Diatomaceoe_.

"2. The second hypothesis is far more feasible, and is strongly supported by the fact that many _Polycistineoe [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 South-Western 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 _Globigerina_ 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 where 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 at 2,000 fathoms is probably, so to speak, very far less than the difference in complexity of organisation 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 _Globigerinoe_ 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 which 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."

[6] [Footnote 6: Appendix to Report on Deep-sea Soundings in the Atlantic Ocean, by Lieut.

-Commander Joseph Dayman.

1857. ] Dr. Wallich, Professor Wyville Thomson, and Dr. Carpenter concluded that the _Globigerinoe_ 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."

[7] Moreover, having obtained certain living star- fish from a depth of 1,260 fathoms, and found their stomachs full of "fresh-looking _Globigerinoe_" and their _débris_--he adduces this fact in support of his belief that the _Globigerinoe_ live at the bottom.

[Footnote 7: The _North Atlantic Sea-bed_, p. 137. ] On the other hand, Müller, Haeckel, Major Owen, Mr. Gwyn Jeffries, and other observers, found that _Globigerinoe_, 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,[8] from which a quotation has already been made, Professor Wyville Thomson says:-- "I had formed and expressed a very strong opinion on the matter.

It seemed to me that the evidence was conclusive that the _Foraminifera_ which formed the _Globigerina_ ooze lived on the bottom, and that the occurrence of individuals on the surface was accidental and exceptional;

but after going into the thing carefully, and considering the mass of evidence which has been accumulated by Mr. Murray, I now admit that I was in error; and I agree with him that it may be taken as proved that all the materials of such deposits, with the exception, of course, of the remains of animals which we now know to live at the bottom at all depths, which occur in the deposit as foreign bodies, are derived from the surface.

[Footnote 8: "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_, Nov.

26, 1874. ] "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 ten to one hundred 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 _Globigerinoe_.

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 further south they are still more dwarfed, and only one variety, the typical _Globigerina bulloides_, is represented.

The living _Globigerinoe_ 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 a 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 _Globigerinoe_ 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 _Globigerinoe_ from the bottom, even in the shallowest water."

There can now be no doubt, therefore, that _Globigerinoe_ live at the top of the sea; but the question may still be raised whether they do not also live at the bottom.

In favour of this view, it has been urged that the shells of the _Globigerinoe_ of the surface never possess such thick walls as those which are fouled at the bottom, but I confess that I doubt the accuracy of this statement.

Again, the occurrence of minute _Globigerinoe_ 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 _Salpoe_ and the like, it is not wonderful that 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 _Globigerinoe_ live at the bottom.

Such as die at the surface, and even many of those which are swallowed by other animals, may retain much of their protoplasmic matter when they reach the depths at which the temperature sinks to 34° or 32° Fahrenheit, where decomposition must become exceedingly slow.

Another consideration appears to me to be in favour of the view that the _Globigerinoe_ 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 anything 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 Faroe 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° N. ; while in the southern seas, the _Globigerinoe_ become dwarfed and almost disappear between 50° and 55° S.

On the other hand, in the sea of Kamschatka, the _Globigerinoe_ have vanished in 56° N. , 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 two thousand 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 one hundred and fifty fathoms, in 31° 32' N. , 79° 35' W.

, off the Coast of Florida, was "a mixture, in about equal proportions, of _Globigerinoe_ and black sand, probably greensand, as it makes a green mark when crushed on paper."

Professor 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 Foraminifer were sornetilnes 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 Professor 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 a hundred to three hundred 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, is a point 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 primitively calcareo- silicious ooze.

The investigation of the deposits which are now being formed in the basin of the Mediterranean, by the late Professor Edward Forbes, by Professor 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 of an excessively fine red clay.