life is dawn on the earth 20

life is dawn on the earth 20

"In connection with these remarkable remains, it appeared desirable to

ascertain, if possible, what share these or other organic structures

may have had in the accumulation of the limestones of the Laurentian

series. Specimens were therefore selected by Sir W. E. Logan, and

slices were prepared under his direction. On microscopic examination,

a number of these were found to exhibit merely a granular aggregation

of crystals, occasionally with particles of graphite and other foreign

minerals, or a laminated mixture of calcareous and other matters, in

the manner of some more modern sedimentary limestones. Others, however,

were evidently made up almost entirely of fragments of Eozoon, or of

mixtures of these with other calcareous and carbonaceous fragments

which afford more or less evidence of organic origin. The contents of

these organic limestones may be considered under the following heads:--

 

1. Remains of Eozoon.

 

2. Other calcareous bodies, probably organic.

 

3. Objects imbedded in the serpentine.

 

4. Carbonaceous matters.

 

5. Perforations, or worm-burrows.

 

"1. The more perfect specimens of Eozoon do not constitute the mass

of any of the larger specimens in the collection of the Survey; but

considerable portions of some of them are made up of material of

similar minute structure, destitute of lamination, and irregularly

arranged. Some of this material gives the impression that there may

have been organisms similar to Eozoon, but growing in an irregular

or acervuline manner without lamination. Of this, however, I cannot

be certain; and on the other hand there is distinct evidence of the

aggregation of fragments of Eozoon in some of these specimens. In

some they constitute the greater part of the mass. In others they

are embedded in calcareous matter of a different character, or in

serpentine or granular pyroxene. In most of the specimens the cells of

the fossils are more or less filled with these minerals; and in some

instances it would appear that the calcareous matter of fragments of

Eozoon has been in part replaced by serpentine."

 

"2. Intermixed with the fragments of Eozoon above referred to, are

other calcareous matters apparently fragmentary. They are of various

angular and rounded forms, and present several kinds of structure. The

most frequent of these is a strong lamination varying in direction

according to the position of the fragments, but corresponding, as

far as can be ascertained, with the diagonal of the rhombohedral

cleavage. This structure, though crystalline, is highly characteristic

of crinoidal remains when preserved in altered limestones. The more

dense parts of Eozoon, destitute of tubuli, also sometimes show this

structure, though less distinctly. Other fragments are compact and

structureless, or show only a fine granular appearance; and these

sometimes include grains, patches, or fibres of graphite. In Silurian

limestones, fragments of corals and shells which have been partially

infiltrated with bituminous matter, show a structure like this. On

comparison with altered organic limestones of the Silurian system,

these appearances would indicate that in addition to the debris of

Eozoon, other calcareous structures, more like those of crinoids,

corals, and shells, have contributed to the formation of the

Laurentian limestones.

 

"3. In the serpentine[AE] filling the chambers of a large specimen of

Eozoon from Burgess, there are numerous small pieces of foreign matter;

and the silicate itself is laminated, indicating its sedimentary

nature. Some of the included fragments appear to be carbonaceous,

others calcareous; but no distinct organic structure can be detected

in them. There are, however, in the serpentine, many minute silicious

grains of a bright green colour, resembling green-sand concretions;

and the manner in which these are occasionally arranged in lines and

groups, suggests the supposition that they may possibly be casts of

the interior of minute Foraminiferal shells. They may, however, be

concretionary in their origin.

 

[Footnote AE: This is the dark green mineral named loganite by Dr.

Hunt.]

 

"4. In some of the Laurentian limestones submitted to me by Sir W.

E. Logan, and in others which I collected some years ago at Madoc,

Canada West, there are fibres and granules of carbonaceous matter,

which do not conform to the crystalline structure, and present forms

quite similar to those which in more modern limestones result from

the decomposition of algæ. Though retaining mere traces of organic

structure, no doubt would be entertained as to their vegetable origin

if they were found in fossiliferous limestones.

 

"5. A specimen of impure limestone from Madoc, in the collection of

the Canadian Geological Survey, which seems from its structure to

have been a finely laminated sediment, shows perforations of various

sizes, somewhat scalloped at the sides, and filled with grains of

rounded silicious sand. In my own collection there are specimens

of micaceous slate from the same region, with indications on their

weathered surfaces of similar rounded perforations, having the aspect

of Scolithus, or of worm-burrows.

 

"Though the abundance and wide distribution of Eozoon, and the

important part it seems to have acted in the accumulation of limestone,

indicate that it was one of the most prevalent forms of animal

existence in the seas of the Laurentian period, the non-existence of

other organic beings is not implied. On the contrary, independently of

the indications afforded by the limestones themselves, it is evident

that in order to the existence and growth of these large Rhizopods, the

waters must have swarmed with more minute animal or vegetable organisms

on which they could subsist. On the other hand, though this is a less

certain inference, the dense calcareous skeleton of Eozoon may indicate

that it also was liable to the attacks of animal enemies. It is also

possible that the growth of Eozoon, or the deposition of the serpentine

and pyroxene in which its remains have been preserved, or both, may

have been connected with certain oceanic depths and conditions, and

that we have as yet revealed to us the life of only certain stations

in the Laurentian seas. Whatever conjectures we may form on these more

problematic points, the observations above detailed appear to establish

the following conclusions:--

 

"First, that in the Laurentian period, as in subsequent geological

epochs, the Rhizopods were important agents in the accumulation of

beds of limestone; and secondly, that in this early period these low

forms of animal life attained to a development, in point of magnitude

and complexity, unexampled, in so far as yet known, in the succeeding

ages of the earth's history. This early culmination of the Rhizopods is

in accordance with one of the great laws of the succession of living

beings, ascertained from the study of the introduction and progress of

other groups; and, should it prove that these great Protozoans were

really the dominant type of animals in the Laurentian period, this fact

might be regarded as an indication that in these ancient rocks we may

actually have the records of the first appearance of animal life on our

planet."

 

With reference to the first of the above heads, I have now to state

that it seems quite certain that the upper and younger portions of

the masses of Eozoon often passed into the acervuline form, and the

period in which this change took place seems to have depended on

circumstances. In some specimens there are only a few regular layers,

and then a heap of irregular cells. In other cases a hundred or more

regular layers were formed; but even in this case little groups of

irregular cells occurred at certain points near the surface. This

may be seen in plate III. I have also found some masses clearly not

fragmental which consist altogether of acervuline cells. A specimen

of this kind is represented in fig. 31. It is oval in outline, about

three inches in length, wholly made up of rounded or cylindrical

cells, the walls of which have a beautiful tubular structure, but

there is little or no supplemental skeleton. Whether this is a portion

accidentally broken off from the top of a mass of Eozoon, or a

peculiar varietal form, or a distinct species, it would be difficult

to determine. In the meantime I have described it as a variety,

"_acervulina_," of the species Eozoon Canadense.[AF] Another variety

also, from Petite Nation, shows extremely thin laminæ, closely placed

together and very massive, and with little supplemental skeleton. This

may be allied to the last, and may be named variety "_minor_."

 

[Footnote AF: _Proceedings of Geological Society_, 1875.]

 

[Illustration: Fig. 31. _Acervuline Variety of Eozoon, St. Pierre._

 

(_a._) General form, half natural size. (_b._) Portion of cellular

interior, magnified, showing the course of the tubuli.]

 

All this, however, has nothing to do with the layers of fragments of

Eozoon which are scattered through the Laurentian limestones. In these

the fossil is sometimes preserved in the ordinary manner, with its

cavities filled with serpentine, and the thicker parts of the skeleton

having their canals filled with this substance. In this case the

chambers may have been occupied with serpentine before it was broken

up. At St. Pierre there are distinct layers of this kind, from half an

inch to several inches in thickness, regularly interstratified with

the ordinary limestone. In other layers no serpentine occurs, but the

interstices of the fragments are filled with crystalline dolomite or

magnesian limestone, which has also penetrated the canals; and there

are indications, though less manifest, that some at least of the layers

of pure limestone are composed of fragmental Eozoon. In the Laurentian

limestone of Wentworth, belonging apparently to the same band with

that of St. Pierre, there are many small rounded pieces of limestone,

evidently the debris of some older rock, broken up and rounded by

attrition. In some of these fragments the structure of Eozoon may be

plainly perceived. This shows that still older limestones composed of

Eozoon were at that time undergoing waste, and carries our view of the

existence of this fossil back to the very beginning of the Laurentian.

 

With respect to organic fragments not showing the structure of Eozoon,

I have not as yet been able to refer these to any definite origin. Some

of them may be simply thick portions of the shell of Eozoon with their

pores filled with calcite, so as to present a homogeneous appearance.

Others have much the appearance of fragments of such Primordial forms

as _Archæocyathus_, to be described in the sequel; but after much

careful search, I have thus far been unable to say more than I could say in 1865.