life is dawn on the earth 15

life is dawn on the earth 15

I may mention here a peculiar and interesting structure which has

been detected in one of my specimens while these sheets were passing

through the press. It is an abnormal thickening of the calcareous

wall, extending across several layers, and perforated with large

parallel cylindrical canals, filled with dolomite, and running in

the direction of the laminæ; the intervening calcite being traversed

by a very fine and delicate canal system. It makes a nearer approach

to some of the Stromatoporæ mentioned in Chapter VI. than any other

Laurentian structure hitherto observed, and may be either an abnormal

growth of Eozoon, consequent on some injury, or a parasitic mass of

some Stromatoporoid organism overgrown by the laminæ of the fossil.

The structure of the dolomite in this specimen indicates that it

first lined the canals, and afterward filled them; an appearance

which I have also observed recently in the larger canals filled

with serpentine (Plate VIII., fig. 5). The cut below is an attempt,

only partially successful, to show the Amœba-like appearance, when

magnified, of the casts of the chambers of Eozoon, as seen on the

decalcified surface of a specimen broken parallel to the laminæ.

 

[Illustration: Fig. 21_a_.]

 

[Illustration:

Plate V.

 

_Nature-print of Eozoon, showing laminated, acervuline, and fragmental

portions._

 

This is printed from an electrotype taken from an etched slab of

Eozoon, and not touched with a graver except to remedy some accidental

flaws in the plate. The diagonal white line marks the course of a

calcite vein.]

 

 

 

 

CHAPTER V.

 

THE PRESERVATION OF EOZOON.

 

 

Perhaps nothing excites more scepticism as to this ancient fossil

than the prejudice existing among geologists that no organism can be

preserved in rocks so highly metamorphic as those of the Laurentian

series. I call this a prejudice, because any one who makes the

microscopic structure of rocks and fossils a special study, soon learns

that fossils undergo the most remarkable and complete chemical changes

without losing their minute structure, and that calcareous rocks if

once fossiliferous are hardly ever so much altered as to lose all

trace of the organisms which they contained, while it is a most common

occurrence to find highly crystalline rocks of this kind abounding in

fossils preserved as to their minute structure.

 

Let us, however, look at the precise conditions under which this takes

place.

 

When calcareous fossils of irregular surface and porous or cellular

texture, such as Eozoon was or corals were and are, become imbedded

in clay, marl, or other soft sediment, they can be washed out and

recovered in a condition similar to that of recent specimens, except

that their pores or cells if open may be filled with the material of

the matrix, or if not so open that they can be thus filled, they may be

more or less incrusted with mineral deposits introduced by water, or

may even be completely filled up in this way. But if such fossils are

contained in hard rocks, they usually fail, when these are broken, to

show their external surfaces, and, breaking across with the containing

rock, they exhibit their internal structure merely,--and this more

or less distinctly, according to the manner in which their cells or

cavities have been filled. Here the microscope becomes of essential

service, especially when the structures are minute. A fragment of

fossil wood which to the naked eye is nothing but a dark stone, or a

coral which is merely a piece of gray or coloured marble, or a specimen

of common crystalline limestone made up originally of coral fragments,

presents, when sliced and magnified, the most perfect and beautiful

structure. In such cases it will be found that ordinarily the original

substance of the fossil remains, in a more or less altered state. Wood

may be represented by dark lines of coaly matter, or coral by its

white or transparent calcareous laminæ; while the material which has

been introduced and which fills the cavities may so differ in colour,

transparency, or crystalline structure, as to act differently on

light, and so reveal the structure. These fillings are very curious.

Sometimes they are mere earthy or muddy matter. Sometimes they are

pure and transparent and crystalline. Often they are stained with

oxide of iron or coaly matter. They may consist of carbonate of lime,

silica or silicates, sulphate of baryta, oxides of iron, carbonate of

iron, iron pyrite, or sulphides of copper or lead, all of which are

common materials. They are sometimes so complicated that I have seen

even the minute cells of woody structures, each with several bands of

differently coloured materials deposited in succession, like the coats

of an onyx agate.

 

A further stage of mineralization occurs when the substance of the

organism is altogether removed and replaced by foreign matter, either

little by little, or by being entirely dissolved or decomposed,

leaving a cavity to be filled by infiltration. In this state are some

silicified woods, and those corals which have been not filled with but

converted into silica, and can thus sometimes be obtained entire and

perfect by the solution in an acid of the containing limestone, or by

its removal in weathering. In this state are the beautiful silicified

corals obtained from the corniferous limestone of Lake Erie. It may be

well to present to the eye these different stages of fossilization. I

have attempted to do this in fig. 22, taking a tabulate coral of the

genus Favosites for an example, and supposing the materials employed to

be calcite and silica. Precisely the same illustration would apply to a

piece of wood, except that the cell-wall would be carbonaceous matter

instead of carbonate of lime. In this figure the dotted parts represent

carbonate of lime, the diagonally shaded parts silica or a silicate.

Thus we have, in the natural state, the walls of carbonate of lime

and the cavities empty. When fossilized the cavities may be merely

filled with carbonate of lime, or they may be filled with silica; or

the walls themselves may be replaced by silica and the cavities may

remain filled with carbonate of lime; or both the walls and cavities

may be represented by or filled with silica or silicates. The ordinary

specimens of Eozoon are in the third of these stages, though some exist

in the second, and I have reason to believe that some have reached to

the fifth. I have not met with any in the fourth stage, though this is

not uncommon in Silurian and Devonian fossils.

 

[Illustration: Fig. 22. _Diagram showing different States of

Fossilization of a Cell of a Tabulate Coral._

 

(_a._) Natural condition--walls calcite, cell empty. (_b._) Walls

calcite, cell filled with the same. (_c._) Walls calcite, cell filled

with silica or silicate. (_d._) Walls silicified, cell filled with

calcite. (_e._) Walls silicified, cell filled with silica or silicate.]

 

With regard to the calcareous organisms with which we have now to do,

when these are imbedded in pure limestone and filled with the same, so

that the whole rock, fossils and all, is identical in composition, and

when metamorphic action has caused the whole to become crystalline,

and perhaps removed the remains of carbonaceous matter, it may be very

difficult to detect any traces of fossils. But even in this case

careful management of light may reveal indications of structure, as in

some specimens of Eozoon described by the writer and Dr. Carpenter. In

many cases, however, even where the limestones have become perfectly

crystalline, and the cleavage planes cut freely across the fossils,

these exhibit their forms and minute structure in great perfection.

This is the case in many of the Lower Silurian limestones of Canada,

as I have elsewhere shown.[X] The gray crystalline Trenton limestone

of Montreal, used as a building stone, is an excellent illustration

of this. To the naked eye it is a gray marble composed of cleavable

crystals; but when examined in thin slices, it shows its organic

fragments in the greatest perfection, and all the minute structures

are perfectly marked out by delicate carbonaceous lines. The only

exception in this limestone is in the case of the Crinoids, in which

the cellular structure is filled with transparent calc-spar, perfectly

identical with the original solid matter, so that they appear solid

and homogeneous, and can be recognised only by their external forms.

The specimen represented in fig. 23, is a mass of Corals, Bryozoa, and

Crinoids, and shows these under a low power, as represented in the

figure; but to the naked eye it is merely a gray crystalline limestone.

The specimen represented in fig. 24 shows the Laurentian Eozoon in a

similar state of preservation. It is from a sketch by Dr. Carpenter,

and shows the delicate canals partly filled with calcite as clear and

colourless as that of the shell itself, and distinguishable only by

careful management of the light.

 

[Footnote X: _Canadian Naturalist_, 1859; Microscopic Structure of

Canadian Limestones.]

 

[Illustration: Fig. 23. _Slice of Crystalline Lower Silurian Limestone;

showing Crinoids, Bryozoa, and Corals in fragments._]

 

[Illustration: Fig. 24. _Wall of Eozoon penetrated with Canals. The

unshaded portions filled with Calcite._ (_After Carpenter._)]

 

In the case of recent and fossil Foraminifers, these--when not so

little mineralized that their chambers are empty, or only partially

filled, which is sometimes the case even with Eocene Nummulites

and Cretaceous forms of smaller size,--are very frequently filled

solid with calcareous matter, and as Dr. Carpenter well remarks,

even well preserved Tertiary Nummulites in this state often fail

greatly in showing their structures, though in the same condition

they occasionally show these in great perfection. Among the finest

I have seen are specimens from the Mount of Olives (fig. 19), and

Dr. Carpenter mentions as equally good those of the London clay of

Bracklesham. But in no condition do modern Foraminifera or those of

the Tertiary and Mesozoic rocks appear in greater perfection than when

filled with the hydrous silicate of iron and potash called glauconite,

and which gives by the abundance of its little bottle-green concretions

the name of "green-sand" to formations of this age both in Europe and

America. In some beds of green-sand every grain seems to have been

moulded into the interior of a microscopic shell, and has retained

its form after the frail envelope has been removed. In some cases the

glauconite has not only filled the chambers but has penetrated the