life is dawn on the earth 16

life is dawn on the earth 16

However this may be, the infiltration of the pores of Eozoon with

serpentine and other silicates has evidently been one main means of

the preservation of its structure. When so infiltrated no metamorphism

short of the complete fusion of the containing rock could obliterate

the minutest points of structure; and that such fusion has not

occurred, the preservation in the Laurentian rocks of the most delicate

lamination of the beds shows conclusively; while, as already stated, it

can be shown that the alteration which has occurred might have taken

place at a temperature far short of that necessary to fuse limestone.

Thus has it happened that these most ancient fossils have been

handed down to our time in a state of preservation comparable, as Dr.

Carpenter states, to that of the best preserved fossil Foraminifera

from the more recent formations that have come under his observation in

the course of all his long experience.

 

Let us now look more minutely at the nature of the typical specimens

of Eozoon as originally observed and described, and then turn to those

preserved in other ways, or more or less destroyed and defaced. Taking

a polished specimen from Petite Nation, like that delineated in Plate

V., we find the shell represented by white limestone, and the chambers

by light green serpentine. By acting on the surface with a dilute

acid we etch out the calcareous part, leaving a cast in serpentine

of the cavities occupied by the soft parts; and when this is done in

polished slices these may be made to print their own characters on

paper, as has actually been done in the case of Plate V., which is an

electrotype taken from an actual specimen, and shows both the laminated

and acervuline parts of the fossil. If the process of decalcification

has been carefully executed, we find in the excavated spaces delicate

ramifying processes of opaque serpentine or transparent dolomite, which

were originally imbedded in the calcareous substance, and which are

often of extreme fineness and complexity. (Plate VI. and fig. 10.)

These are casts of the canals which traversed the shell when still

inhabited by the animal. In some well preserved specimens we find the

original cell-wall represented by a delicate white film, which under

the microscope shows minute needle-like parallel processes representing

its still finer tubuli. It is evident that to have filled these tubuli

the serpentine must have been introduced in a state of actual solution,

and must have carried with it no foreign impurities. Consequently we

find that in the chambers themselves the serpentine is pure; and if we

examine it under polarized light, we see that it presents a singularly

curdled or irregularly laminated appearance, which I have designated

under the name septariiform, as if it had an imperfectly crystalline

structure, and had been deposited in irregular laminæ, beginning at

the sides of the chambers, and filling them toward the middle, and

had afterward been cracked by shrinkage, and the cracks filled with a

second deposit of serpentine. Now, serpentine is a hydrous silicate of

magnesia, and all that we need to suppose is that in the deposits of

the Laurentian sea magnesia was present instead of iron and potash,

and we can understand that the Laurentian fossil has been petrified

by infiltration with serpentine, as more modern Foraminifera have

been with glauconite, which, though it usually has little magnesia,

often has a considerable percentage of alumina. Further, in specimens

of Eozoon from Burgess, the filling mineral is loganite, a compound

of silica, alumina, magnesia and iron, with water, and in certain

Silurian limestones from New Brunswick and Wales, in which the delicate

microscopic pores of the skeletons of stalked star-fishes or Crinoids

have been filled with mineral deposits, so that when decalcified

these are most beautifully represented by their casts, Dr. Hunt has

proved the filling mineral to be a silicate of alumina, iron, magnesia

and potash, intermediate between serpentine and glauconite. We have,

therefore, ample warrant for adhering to Dr. Hunt's conclusion that

the Laurentian serpentine was deposited under conditions similar to

those of the modern green-sand. Indeed, independently of Eozoon, it is

impossible that any geologist who has studied the manner in which this

mineral is associated with the Laurentian limestones could believe it

to have been formed in any other way. Nor need we be astonished at

the fineness of the infiltration by which these minute tubes, perhaps

1/10000 of an inch in diameter, are filled with mineral matter. The

micro-geologist well knows how, in more modern deposits, the finest

pores of fossils are filled, and that mineral matter in solution

can penetrate the smallest openings that the microscope can detect.

Wherever the fluids of the living body can penetrate, there also

mineral substances can be carried, and this natural injection, effected

under great pressure and with the advantage of ample time, can surpass

any of the feats of the anatomical manipulator. Fig. 25 represents

a microscopic joint of a Crinoid from the Upper Silurian of New

Brunswick, injected with the hydrous silicate already referred to, and

fig. 26 shows a microscopic chambered or spiral shell, from a Welsh

Silurian limestone, with its cavities filled with a similar substance.

 

[Illustration: Fig. 25. _Joint of a Crinoid, having its pores injected

with a Hydrous Silicate._

 

Upper Silurian Limestone, Pole Hill, New Brunswick. Magnified 25

diameters.]

 

[Illustration: Fig. 26. _Shell from a Silurian Limestone, Wales; its

cavity filled with a Hydrous Silicate._

 

Magnified 25 diameters.]

 

It is only necessary to refer to the attempts which have been made to

explain by merely mineral deposits the occurrence of the serpentine

in the canals and chambers of Eozoon, and its presenting the form it

does, to see that this is the case. Prof. Rowney, for example, to avoid

the force of the argument from the canal system, is constrained to

imagine that the whole mass has at one time been serpentine, and that

this has been partially washed away, and replaced by calcite. If so,

whence the deposition of the supposed mass of serpentine, which has to

be accounted for in this way as well as in the other? How did it happen

to be eroded into so regular chambers, leaving intermediate floors and

partitions. And, more wonderful still, how did the regular dendritic

bundles, so delicate that they are removed by a breath, remain perfect,

and endure until they were imbedded in calcareous spar? Further, how

does it happen that in some specimens serpentine and pyroxene seem to

have encroached upon the structure, as if they and not calcite were the

eroding minerals? How any one who has looked at the structures can for

a moment imagine such a possibility, it is difficult to understand. If

we could suppose the serpentine to have been originally deposited as

a cellular or laminated mass, and its cavities filled with calcite in

a gelatinous or semi-fluid state, we might suppose the fine processes

of serpentine to have grown outward into these cavities in the mass,

as fibres of oxide of iron or manganese have grown in the silica of

moss-agate; but this theory would be encompassed with nearly as great

mechanical and chemical difficulties. The only rational view that any

one can take of the process is, that the calcareous matter was the

original substance, and that it had delicate tubes traversing it which

became injected with serpentine. The same explanation, and no other,

will suffice for those delicate cell-walls, penetrated by innumerable

threads of serpentine, which must have been injected into pores. It is

true that there are in some of the specimens cracks filled with fibrous

serpentine or chrysotile, but these traverse the mass in irregular

directions, and they consist of closely packed angular prisms,

instead of a matrix of limestone penetrated by cylindrical threads of

serpentine. (Fig. 27.) Here I must once for all protest against the

tendency of some opponents of Eozoon to confound these structures and

the canal system of Eozoon with the acicular crystals, and dendritic

or coralloidal forms, observed in some minerals. It is easy to make

such comparisons appear plausible to the uninitiated, but practised

observers cannot be so deceived, the differences are too marked and

essential. In illustration of this, I may refer to the highly magnified

canals in figs. 28 and 29. Further, it is evident from the examination

of the specimens, that the chrysotile veins, penetrating as they often

do diagonally or transversely across both chambers and walls, must have

originated subsequently to the origin and hardening of the rock and its

fossils, and result from aqueous deposition of fibrous serpentine in

cracks which traverse alike the fossils and their matrix. In specimens

now before me, nothing can be more plain than this entire independence

of the shining silky veins of fibrous serpentine, and the fact of their

having been formed subsequently to the fossilization of the Eozoon;

since they can be seen to run across the lamination, and to branch off

irregularly in lines altogether distinct from the structure. This,

while it shows that these veins have no connection with the fossil,

shows also that the latter was an original ingredient of the beds when

deposited, and not a product of subsequent concretionary action.

 

[Illustration: Fig. 27. _Diagram showing the different appearances

of the cell-wall of Eozoon and of a vein of Chrysotile, when highly

magnified._]

 

[Illustration: Fig. 28. _Casts of Canals of Eozoon in Serpentine,

decalcified and highly magnified._]

 

[Illustration: Fig. 29. _Canals of Eozoon._

 

Highly magnified.]

 

Taking the specimens preserved by serpentine as typical, we now turn

to certain other and, in some respects, less characteristic specimens,

which are nevertheless very instructive. At the Calumet some of

the masses are partly filled with serpentine and partly with white

pyroxene, an anhydrous silicate of lime and magnesia. The two minerals

can readily be distinguished when viewed with polarized light; and in

some slices I have seen part of a chamber or group of canals filled

with serpentine and part with pyroxene. In this case the pyroxene

or the materials which now compose it, must have been introduced by

infiltration, as well as the serpentine. This is the more remarkable as

pyroxene is most usually found as an ingredient of igneous rocks; but

Dr. Hunt has shown that in the Laurentian limestones and also in veins

traversing them, it occurs under conditions which imply its deposition

from water, either cold or warm. Gümbel remarks on this:--"Hunt, in

a very ingenious manner, compares this formation and deposition of

serpentine, pyroxene, and loganite, with that of glauconite, whose

formation has gone on uninterruptedly from the Silurian to the Tertiary

period, and is even now taking place in the depths of the sea; it being

well known that Ehrenberg and others have already shown that many of

the grains of glauconite are casts of the interior of foraminiferal

shells. In the light of this comparison, the notion that the serpentine

and such like minerals of the primitive limestones have been formed,

in a similar manner, in the chambers of Eozoic Foraminifera, loses any

traces of improbability which it might at first seem to possess."

 

In many parts of the skeleton of Eozoon, and even in the best

infiltrated serpentine specimens, there are portions of the cell-wall