life is dawn on the earth 17

life is dawn on the earth 17

Two-thirds natural size. (_a._) Tubuli. (_b._) Canals. Magnified. _a_

and _b_ from another specimen.]

 

Still another mode of occurrence is presented by a remarkable specimen

from Tudor in Ontario, and from beds probably on the horizon of the

Upper Laurentian or Huronian.[Y] It occurs in a rock scarcely at all

metamorphic, and the fossil is represented by white carbonate of lime,

while the containing matrix is a dark-coloured coarse limestone. In

this specimen the material filling the chambers has not penetrated

the canals except in a few places, where they appear filled with dark

carbonaceous matter. In mode of preservation these Tudor specimens

much resemble the ordinary fossils of the Silurian rocks. One of

the specimens in the collection of the Geological Survey (fig. 30)

presents a clavate form, as if it had been a detached individual

supported on one end at the bottom of the sea. It shows, as does

also the original Calumet specimen, the septa approaching each other

and coalescing at the margin of the form, where there were probably

orifices communicating with the exterior. Other specimens of fragmental

Eozoon from the Petite Nation localities have their canals filled with

dolomite, which probably penetrated them after they were broken up

and imbedded in the rock. I have ascertained with respect to these

fragments of Eozoon, that they occur abundantly in certain layers of

the Laurentian limestone, beds of some thickness being in great part

made up of them, and coarse and fine fragments occur in alternate

layers, like the broken corals in some Silurian limestones.

 

[Footnote Y: See Note B, Chap. III.]

 

Finally, on this part of the subject, careful observation of many

specimens of Laurentian limestone which present no trace of Eozoon

when viewed by the naked eye, and no evidence of structure when acted

on with acids, are nevertheless organic, and consist of fragments

of Eozoon, and possibly of other organisms, not infiltrated with

silicates, but only with carbonate of lime, and consequently revealing

only obscure indications of their minute structure. I have satisfied

myself of this by long and patient investigations, which scarcely admit

of any adequate representation, either by words or figures.

 

Every worker in those applications of the microscope to geological

specimens which have been termed micro-geology, is familiar with the

fact that crystalline forces and mechanical movements of material

often play the most fantastic tricks with fossilized organic matter.

In fossil woods, for example, we often have the tissues disorganized,

with radiating crystallizations of calcite and little spherical

concretions of quartz, or disseminated cubes and grains of pyrite,

or little veins filled with sulphate of barium or other minerals. We

need not, therefore, be surprised to find that in the venerable rocks

containing Eozoon, such things occur in the more highly crystalline

parts of the limestones, and even in some still showing traces of

the fossil. We find many disseminated crystals of magnetite, pyrite,

spinel, mica, and other minerals, curiously curved prisms of vermicular

mica, bundles of aciculi of tremolite and similar substances, veins of

calcite and crysolite or fibrous serpentine, which often traverse the

best specimens. Where these occur abundantly we usually find no organic

structures remaining, or if they exist they are in a very defective

state of preservation. Even in specimens presenting the lamination of

Eozoon to the naked eye, these crystalline actions have often destroyed

the minute structure; and I fear that some microscopists have been

victimised by having under their consideration only specimens in which

the actual characters had been too much defaced to be discernible. I

must here state that I have found some of the specimens sold under

the name of Eozoon Canadense by dealers in microscopical objects to

be almost or quite worthless, being destitute of any good structure,

and often merely pieces of Laurentian limestone with serpentine

grains only. I fear that the circulation of such specimens has done

much to cause scepticism as to the Foraminiferal nature of Eozoon. No

mistake can be greater than to suppose that any and every specimen

of Laurentian limestone must contain Eozoon. More especially have

I hitherto failed to detect traces of it in those carbonaceous or

graphitic limestones which are so very abundant in the Laurentian

country. Perhaps where vegetable matter was very abundant Eozoon

did not thrive, or on the other hand the growth of Eozoon may have

diminished the quantity of vegetable matter. It is also to be observed

that much compression and distortion have occurred in the beds of

Laurentian limestone and their contained fossils, and also that the

specimens are often broken by faults, some of which are so small as to

appear only on microscopic examination, and to shift the plates of the

fossil just as if they were beds of rock. This, though it sometimes

produces puzzling appearances, is an evidence that the fossils were

hard and brittle when this faulting took place, and is consequently

an additional proof of their extraneous origin. In some specimens it

would seem that the lower and older part of the fossil had been wholly

converted into serpentine or pyroxene, or had so nearly experienced

this change that only small parts of the calcareous wall can be

recognised. These portions correspond with fossil woods altogether

silicified, not only by the filling of the cells, but also by the

conversion of the walls into silica. I have specimens which manifestly

show the transition from the ordinary condition of filling with

serpentine to one in which the cell-walls are represented obscurely by

one shade of this mineral and the cavities by another.

 

The above considerations as to mode of preservation of Eozoon concur

with those in previous chapters in showing its oceanic character;

but the ocean of the Eozoic period may not have been so deep as at

present, and its waters were probably warm and well stocked with

mineral matters derived from the newly formed land, or from hot springs

in its own bottom. On this point the interesting investigations of

Dr. Hunt with reference to the chemical conditions of the Silurian

seas, allow us to suppose that the Laurentian ocean may have been much

more richly stored, more especially with salts of lime and magnesia,

than that of subsequent times. Hence the conditions of warmth, light,

and nutriment, required by such gigantic Protozoans would all be

present, and hence, also no doubt, some of the peculiarities of its

mineralization.

 

 

NOTES TO CHAPTER V.

 

 

(A.) Dr. Sterry Hunt on the Mineralogy of Eozoon and the containing

Rocks.

 

It was fortunate for the recognition of Eozoon that Dr. Hunt had,

before its discovery, made so thorough researches into the chemistry

of the Laurentian series, and was prepared to show the chemical

possibilities of the preservation of fossils in these ancient

deposits. The following able summary of his views was appended to the

original description of the fossil in the _Journal of the Geological

Society_.

 

"The details of structure have been preserved by the introduction

of certain mineral silicates, which have not only filled up the

chambers, cells, and canals left vacant by the disappearance of the

animal matter, but have in very many cases been injected into the

tubuli, filling even their smallest ramifications. These silicates

have thus taken the place of the original sarcode, while the

calcareous septa remain. It will then be understood that when the

replacement of the Eozoon by silicates is spoken of, this is to be

understood of the soft parts only; since the calcareous skeleton is

preserved, in most cases, without any alteration. The vacant spaces

left by the decay of the sarcode may be supposed to have been filled

by a process of infiltration, in which the silicates were deposited

from solution in water, like the silica which fills up the pores of

wood in the process of silicification. The replacing silicates, so

far as yet observed, are a white pyroxene, a pale green serpentine,

and a dark green alumino-magnesian mineral, which is allied in

composition to chlorite and to pyrosclerite, and which I have

referred to loganite. The calcareous septa in the last case are found

to be dolomitic, but in the other instances are nearly pure carbonate

of lime. The relations of the carbonate and the silicates are well

seen in thin sections under the microscope, especially by polarized

light. The calcite, dolomite, and pyroxene exhibit their crystalline

structure to the unaided eye; and the serpentine and loganite are

also seen to be crystalline when examined with the microscope. When

portions of the fossil are submitted to the action of an acid, the

carbonate of lime is dissolved, and a coherent mass of serpentine is

obtained, which is a perfect cast of the soft parts of the Eozoon.

The form of the sarcode which filled the chambers and cells is

beautifully shown, as well as the connecting canals and the groups

of tubuli; these latter are seen in great perfection upon surfaces

from which the carbonate of lime has been partially dissolved. Their

preservation is generally most complete when the replacing mineral is

serpentine, although very perfect specimens are sometimes found in

pyroxene. The crystallization of the latter mineral appears, however,

in most cases to have disturbed the calcareous septa.

 

"Serpentine and pyroxene are generally associated in these specimens,

as if their disposition had marked different stages of a continuous

process. At the Calumet, one specimen of the fossil exhibits the

whole of the sarcode replaced by serpentine; while, in another one

from the same locality, a layer of pale green translucent serpentine

occurs in immediate contact with the white pyroxene. The calcareous

septa in this specimen are very thin, and are transverse to the plane

of contact of the two minerals; yet they are seen to traverse both

the pyroxene and the serpentine without any interruption or change.

Some sections exhibit these two minerals filling adjacent cells,

or even portions of the same cell, a clear line of division being

visible between them. In the specimens from Grenville on the other

hand, it would seem as if the development of the Eozoon (considerable

masses of which were replaced by pyroxene) had been interrupted, and

that a second growth of the animal, which was replaced by serpentine,

had taken place upon the older masses, filling up their interstices."