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.
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.