that the laminæ of the test remain as calcite, while the chambers and
larger canals are filled with serpentine of a light green or olive
colour, and the finer tubuli are injected with dolomite. It may also
be observed that the serpentine in the larger cavities often shows a
banded structure, as if it had been deposited in successive coats, and
the canals are sometimes lined with a tubular film of serpentine, with
a core or axis of dolomite, which also extends into the finer tubuli of
the surfaces of the laminæ. This, on the theory of animal origin, is
the most perfect state of preservation, and it equals anything I have
seen in calcareous organisms of later periods. This state of perfection
is, however, naturally of infrequent occurrence.
[Illustration: Fig. 37.--_Finest Tubuli filled with Dolomite
(magnified)._]
The finer tubuli are rarely perfect or fully infiltrated. Even the
coarser canals are not infrequently imperfect, while the laminæ
themselves are sometimes crumpled, crushed, faulted, or penetrated with
veins of chrysotile or of calcite. In some instances the calcareous
laminæ are replaced by dolomite, in which case the canal-systems
are always imperfect or obsolete. The laminæ of the test itself are
also in some cases replaced by serpentine in a flocculent form. At
the opposite extreme are specimens, or portions of specimens, in
which the chambers are obliterated by pressure, or occupied only
with calcite. In such cases the general structure is entirely lost
to view, and scarcely appears in weathering. It can be detected only
by microscopic examination of slices, in parts where the granular
structure or the tubulation of the calcite layers has been preserved.
All palæontologists who have studied silicified fossils in the older
rocks are familiar with such appearances.
[Illustration: Fig. 38.--_Plan of arrangement of Canals in Lamina of
Eozoon._]
It has been alleged by Möbius and others that the canal-systems and
tubes present no organic regularity. This difficulty, however, arises
solely from imperfect specimens or inattention to the necessary results
of slicing any system of ramifying canals. In Eozoon the canals form
ramifying groups in the middle planes of the laminæ, and proceed at
first almost horizontally, dividing into smaller branches, which
ultimately give off brushes of minute tubuli running nearly at right
angles to the surfaces of the lamina, and forming the extremely fine
tubulation which Dr. Carpenter regarded as the proper wall (Figs. 38,
39).
[Illustration: Fig. 39.--_Cross section of minute Tubuli, about 5
microms in diameter (magnified)._]
In my earlier description I did not distinguish this from the
canal-system, with which its tubuli are inwardly continuous. Dr.
Carpenter, however, understood this arrangement, and has represented
it in his figures[30] (see also Fig. 28). It is evident that in a
structure like this a transverse or oblique section will show truncated
portions of the larger tubes apparently intermixed with others much
finer and not continuous with them, except very rarely. Good specimens
and many slices and decalcified portions are necessary to understand
the arrangement This consideration alone, I think, entirely invalidates
the criticisms of Möbius, and renders his large and costly figures of
little value, though his memoir is, as I have elsewhere shown, liable
to other and fatal objections.[31]
[Footnote 30: "Ann. and Mag. Nat. Hist.," ser. 4, xiii., p. 456, figs.
3, 4.]
[Footnote 31: "Museum Memoir," pp. 50 _et seq._]
It has been pretended that the veins of chrysotile, when parallel
to the laminæ, cannot be distinguished from the minute tubuli
terminating on the surfaces of the laminæ. I feel confident, however,
that no microscopist who has seen both, under proper conditions of
preservation and study, could confound them. The fibres of chrysotile
are closely appressed parallel prisms, with the optical properties of
serpentine. The best preserved specimens of the "proper wall" contain
no serpentine, but are composed of calcite with extremely minute
parallel cylinders of dolomite about five to ten microms. in diameter,
and separated by spaces greater than their own diameter (Figs. 40, 41).
In the rare cases where the cylinders are filled with serpentine, they
are, of course, still more distinct and beautiful. At the same time,
I do not doubt that observers who have not seen the true tubulation
may have been misled by chrysotile veins when these fringe the laminæ.
Möbius, for instance, figures the true and false structure as if they
were the same.
[Illustration: Fig. 40.--_Cross section of similar Tubuli to those in
Fig. 39, more highly magnified, and showing granular character of the
test._
(From camera tracings.)]
[Illustration: Fig. 41.--_Comparison of Tubulate Wall and Prisms of
Chrysotile in perspective._]
[Illustration: _Canals of Eozoon._ (After Möbius.)]
[Illustration: _Finer Canals of Eozoon._ (After Möbius.)]
[Illustration: _Canals of modern Calcarina._ (After Carpenter.)]
[Illustration: _Canals and Tubule of Tertiary Nummulina._ (After
Möbius.)]
Fig. 42.
Figures selected from Möbius, to show the resemblance of structures of
Eozoon to those of modern Foraminifera.
Protest should here be made against that mode of treating ancient
fossils which regards the most obscure or defaced specimens as typical,
and those better preserved as mere accidents, of mineral structure. In
Tertiary Nummulites injected with glauconite it is rare to find the
tubuli perfectly filled, except in tufts here and there; yet no one
doubts that these patches represent a continuous structure.
I have remarked on previous occasions that the calcite constituting the
laminæ of Eozoon often has a minutely granular appearance, different
from that of the surrounding limestone. Under a high power it resolves
itself into extremely minute dots or flocculi, somewhat uniformly
diffused. Whether these dots are particles of carbon, iron, apatite, or
silicious matter, or the remains of a porous structure, I do not know;
but similar appearances occur in the calcareous fossils contained in
altered limestones of later date. Wherever they occur in crystalline
limestones, supposed to be organic, the microscopist should examine
them with care. I have sometimes by this appearance detected fragments
of Eozoon which afterward revealed their canals.
(2) The second question requires us to consider the nature and
origin of the substances constituting the specimens. Reference has
already been made to these in our fifth chapter, but they may be
more particularly noticed here in connection with the forms as above
described.
The calcareous laminæ are usually composed of clear translucent
calcite or calcium carbonate, though, as in the case of many later
fossils, sometimes replaced by dolomite. It often has the fine granular
appearance above referred to, but is nearly always crystalline, and
traversed by cleavage planes visible under the microscope.[32] This
crystalline structure, as every student of fossils knows, is very
common in calcareous fossils of all geological ages. In the thicker
laminæ the canals traversing them and branching out in their substance
are usually visible under a low power, except when they are filled with
calcite similar to that of the laminæ themselves. In this case they
can be seen only by very careful management of an oblique and subdued
light. When occupied with serpentine, this presents, in a thin slice
under transmitted light, a yellowish or brownish colour, and in a
specimen decalcified with an acid an opaque white appearance. In some
of the larger threads of serpentine, as already stated, this mineral
forms a thin outer cylinder with a core of calcite or dolomite within;
but this appearance is not common. Here and there, especially in the
lower layers, a portion of a tube is filled with the harder mineral
pyroxene, which is in some respects similar to serpentine, except
that it contains lime as well as magnesia, and is destitute of water
as an ingredient The finer tubuli into which the canals ramify are
most usually filled with dolomite or magnesian limestone, which has
a glossy appearance and higher lustre than the surrounding calcite,
and so may be distinguished even in a transparent slice; but these
fine dolomite threads are best seen when the surface of a slice is
treated with a dilute acid in the cold, in which circumstances the
calcite is dissolved, while the dolomite remains as tufts of delicate
cylindrical hairs, presenting often a very beautiful appearance under
the microscope. Thus, as in many other fossils, what are supposed to
have been tubes and tubuli are found not empty, but filled with matter
even harder and more resisting than the shell itself.
[Footnote 32: Especially when the specimen has been heated or jarred in
the process of grinding or polishing.]
Serpentine is a mineral which has been produced in different ways.
Some igneous or volcanic rocks consist largely of compounds of silica
and magnesia (olivine, etc.). When these rocks have become cold and
are exposed to the action of water, they sometimes absorb this and
become hydrated, thus passing into a kind of serpentine. When such
rocks are pulverized and dispersed as volcanic ash, this falling into
the sea may be there hydrated, and may form serpentinous layers, or in
a fine paste or in solution may pass into the pores and cavities of
shells and other organic things, acting, as we have seen, in the same
manner with ordinary glauconite. In like manner serpentine of this
origin may form nodules or grains in limestones, in consequence of its
particles being aggregated together by concretionary attraction. We
have already seen that some comparatively modern so-called glauconites
are essentially of the nature of serpentine, and we know that in the
old Laurentian sea, salts of magnesia and magnesian minerals were
abundant, so that serpentinous minerals might play a greater part than
they do in the modern seas. Loganite, the mineralizing substance of
the Burgess Eozoon, is different from serpentine, yet closely allied
to the glauconites. The presence of pyroxene may be explained in a
similar way. It is a frequent constituent of bedded volcanic rocks
and of volcanic ashes, and beds of it occur in the Grenville series
which once, no doubt, were ash-beds. Layers of it also occasionally
occur from a similar cause in the limestone, and crystals of it have
been deposited by water in the veins passing through the limestones
and schists. Dr. Johnston-Lavis has described in the July number of
the _Geological Magazine_ for 1895 the aqueous deposition at ordinary
temperature of crystals of pyroxene and hornblende, in cavities and
crevices of bones included in an ash-bed of recent date, and in
presence of calcite, apatite, and fluoride of calcium, as in the
Grenville series. This is a modern instance analogous to that suggested
above. Hence all these minerals filling the cavities and canals of
Eozoon may have been deposited by water at ordinary temperatures, and
have no connection with the alteration to which the beds have been
subsequently subjected.
I may add here that a Tertiary glauconite from the Calcaire Grossier
of Paris analysed by Berthier[33] is essentially a serpentine composed
of silicate of iron and magnesia, that Loganite as analysed by Hunt
contains thirty-one per cent, of magnesia, and that Hoskins has
shown[34] that modern glauconites often contain large proportions of
magnesia and equivalent bases.
[Footnote 33: Beudant, _Mineralogie_, xi. 178.]
[Footnote 34: _Geological Magazine_, July, 1895.]
It is also to be observed that independently of volcanic debris the
reports of the _Challenger_ expedition show that in the deep seas
the decay of organic matter causes an alkaline condition of the
sediments leading to the formation of alkaline silicates, while the
presence of decaying volcanic dust furnishes the basis, whether of
iron, alumina, or magnesia, necessary for the making up of glauconite.
I have also suggested that the assimilation by Protozoa making
calcareous skeletons, of the matter of Diatoms or humble plants having
soluble silica in their organization or of silicious Protozoa, and
sponge germs, must set free much soluble silica as a rejected or
excrementitious matter which may contribute to the same result.
It is much more likely that the serpentine of the Laurentian limestones
was produced in these ways than that it resulted from the hydration
of magnesian minerals after the rock was consolidated. In the former
case it would be in the most favourable conditions for mineralizing
organisms as glauconites do in the modern seas. In the latter it would
cause disturbances and changes of volume of which we have no evidence.
We thus find that the chemistry of the modern seas and that relating to
the preservation of fossils of various ages by silicious infiltrations
lends great probability to the belief that serpentine played this
role in the oldest seas, though it would seem that dolomite was more
suitable to the filling of the extremities of the minute tubes and
their finer terminations.[35]
[Footnote 35: I have shown also that in the limestone containing Eozoon
we find layers holding concretions of serpentine alternating with
others holding crystals of dolomite, as if there were at some times
conditions favourable to the deposition of silicate of magnesia, and at
others to that of the carbonate.]
[Illustration: Fig. 43.--Stromatocerium rugosum, Hall, Ordovician.]
(3) Our third question leads to the inquiry in what modern or ancient
marine animals we can find structures akin to those of our supposed
Laurentian fossil. The first analogy which suggested itself to Sir W.
Logan, and a very natural one, was that to the so-called layer-corals
(Figs. 43 to 45) that abound in the Silurian, Ordovician, and Cambrian
rocks, and which though undoubtedly fossil animals, have proved very
difficult to interpret or to assign to any known group. At first
vaguely associated with the true corals, they were subsequently
regarded as probably of more simple character, and as gigantic
Protozoa; and later strong reasons have been assigned for giving them
an intermediate place, as allied to those curious communities of humble
animals possessing simple stomachs and prehensile tentacles (Hydroids)
which form some of the simpler corals (Millepores, etc.), and the
crusts (Hydractiniæ) which cover dead shells and other bodies in the
sea. When examined microscopically, however, they differ very much
among themselves, and it may be that some of them were Hydroids and
some Protozoa.
[Illustration: Fig. 44.--_Structures of Stromatopora._
(_a_) Portion of oblique section, (_b_) Wall with pores, and coated
with crystals of quartz, (_c_) Thickened portion of wall with canals,
(_d_) Laminæ and pillars.]
[Illustration: Fig. 45.--_Tubular Structure of Cœnostroma, Silurian._]
The oldest that we at present know, and consequently the nearest in
time to Eozoon, impress us rather with the latter affinity. They
are the fossils of the genus Cryptozoon of Hall (Fig. 7[36]), which
form great masses filling certain beds of Upper Cambrian age, and
which, when sliced and studied microscopically, are found to consist
of concentric thin laminæ filled in between with a porous mass of
calcareous matter penetrated by an infinity of tortuous tubes. Forms of
this kind have been traced downward into pre-Cambrian beds in Colorado,
and as we shall find in New Brunswick, into the Upper Laurentian itself.
[Footnote 36: See Figs. 7 and 7a, pp. 37, 38; also Fig. 8 and
Microscopic slice, Fig. 59, at end.]
They present, however, structural differences from Eozoon, which
rather conforms to the arrangements found in some Protozoa of smaller
size, and which, under the name of Foraminifera, have abounded in all
geological periods, and are excessively abundant in the modern ocean.
They may be defined as animals composed of a soft and apparently
homogeneous animal jelly known as protoplasm or sarcode. When carefully
examined, however, it is found to have a granular texture and to be
divisible into two layers, an outer and an inner, while it possesses
a little hollow vessel capable of expanding and absorbing the liquid
matter of the enclosing protoplasm, and of contracting so as to expel
its contents. This seems to be the only organ of circulation and
excretion. There are, however, small cells or reproductive bodies in
the interior, varying in number, size, and development in different
forms. The most remarkable property of these creatures is that of
stretching out from the surface of the body threads or projections of
the protoplasm,[37] often of considerable length, and which serve at
once as organs of locomotion and prehension.
[Footnote 37: Known as Pseudopodia.]
[Illustration: _Amœba._]
[Illustration: _Actinophrys._]
From original sketches.
[Illustration: _Biloculina._ A many-chambered Foraminifer. Magnified as
a transparent object.]
[Illustration: _Polystomella._ A spiral Foraminifer. Magnified as an
opaque object.]
Fig. 46.--Recent Protozoa.
These creatures are in some respects the simplest of animals, yet
in other respects they present strange complexities. This is more
especially evident in their tests or coverings, made for the most part
of limestone or calcium carbonate, but sometimes of grains of fine
sand cemented together. These coverings are always perforated with at
least one orifice for the emission of the thread-like processes or
pseudopods, and often with a vast number of small pores for the same
purpose. Sometimes the test or shell is smooth, sometimes beautifully
sculptured externally. Sometimes it consists of a single chamber like
a ball or vase. More often, as the animals increase in size, they
form additional chambers, and the body thus becomes divided into
lobes connected with each other by necks passing through orifices in
the partitions. The chambers are arranged in rows or in spirals, and
in other ways, giving a vast variety of forms, often presenting the
most beautiful patterns executed in the purest white marble, and the
ornamental parts constitute thickenings of the walls giving greater
strength, and are penetrated with microscopic canals communicating with
the soft substance of the animal.
These creatures abound in all parts of the ocean, from the surface
to the greatest depths. The Foraminifera have also existed from
the earliest geological times, and in all the long ages of the
earth's history seem to have retained the same structures and even
ornamentation; so that species from very old geological formations
are often scarcely distinguishable from those now living, and must
have played precisely the same parts in the system of nature. One of
these functions is that of accumulating great thicknesses of calcareous
matter in the sea-bottom.
The manner in which such accumulation takes place we learn from what
is now going on in the ocean, more especially from the result of the
recent deep-sea dredging expeditions. The Foraminifera are vastly
numerous, both near the surface and at the bottom of the sea, and
multiply rapidly; and as successive generations die, their shells
accumulate on the ocean bed, or are swept by currents into banks, and
thus in process of time constitute thick beds of white chalky material,
which may eventually be hardened into limestone. This process is now
depositing a great thickness of white ooze in the bottom of the ocean;
and in times past it has produced such vast thicknesses of calcareous
matter as the chalk and the nummulitic limestone of Europe and the
orbitoidal limestone of America. The chalk, which alone attains a
maximum thickness of 1,000 feet, and, according to Lyell, can be traced
across Europe for 1,100 geographical miles, may be said to be entirely
composed of shells of Foraminifera imbedded in a paste of still more
minute calcareous bodies, the Coccoliths, which are probably products
of marine vegetable life, if not of some animal organism still simpler
than the Foraminifera.
There are, however, some sessile examples of these animals which
attain to larger dimensions than the free and locomotive forms. As an
example of these we may take the _Polytrema_, which forms little hard
red lumps on West Indian corals. Such a creature, beginning life as a
little round spot of protoplasm, almost invisible, and protected with a
little dome of carbonate of lime for the extension of its pseudopods as
it grows in size, adds chamber to chamber in successive tiers till it
assumes an appreciable size, all the chambers communicating with each
other, while the outer ones are perforated with pores for extension of
the pseudopods. In one form (_Carpenteria_) the same end is secured by
leaving an open space in the middle of the conical mass like the crater
of a small volcano. It is with these larger and sessile forms that
we must compare Eozoon, though some of its minute structures rather
resemble those of some smaller types.
All the creatures referred to above, notwithstanding the differences
in their skeletons, resemble each other very closely in their soft
parts, and come under the general name of Foraminifera, a name
having reference to the openings by which the animal matter within
communicates with the water without, for nutrition and respiration.
Such creatures may be regarded as the simplest and most ready media
for the conversion of vegetable matter into animal tissues, and their
functions are almost entirely limited to those of nutrition. Hence it
is likely that they will be able to appear in the most gigantic forms
under such conditions as afford them the greatest amount of pabulum
for the nourishment of their soft parts and for their skeletons. There
is reason to believe, for example, that the occurrence, both in the
chalk and the deep-sea mud, of immense quantities of the minute oval
bodies known as Coccoliths along with Foraminifera, is not accidental.
The Coccoliths appear to be grains of calcareous matter formed in
minute plants adapted to a deep-sea habitat; and these, along with
the vegetable and animal debris constantly being derived from the
death of the living things at the surface, and falling to the bottom,
afford the material both of sarcode and shell. Now if the Laurentian
graphite represents an exuberance of vegetable growth in those old seas
proportionate to the great supplies of carbonic acid in the atmosphere
and in the waters, and if the Eozoic ocean was even better supplied
with carbonate of lime than those Silurian seas whose vast limestones
bear testimony to their richness in such material, we can easily
imagine that the conditions may have been more favourable to a creature
like Eozoon than those of any other period of geological time.