life is dawn on the earth 25

life is dawn on the earth 25

Contenting myself with these general remarks, I shall, for the benefit

of those who relish geological controversy, append to this chapter a

summary of the objections urged by the most active opponents of the

animal nature of Eozoon, with the replies that may be or have been

given; and I now merely add (in fig. 49) a magnified camera tracing of

a portion of a lamina of Eozoon with its canals and tubuli, to show

more fully the nature of the structures in controversy.

 

[Illustration: Fig. 49. _Portion of a thin Transverse Slice of a Lamina

of Eozoon, magnified, showing its structure, as traced with the camera._

 

(_a._) Nummuline wall of under side. (_b._) Intermediate skeleton with

canals. (_a´._) Nummuline wall of upper side. The two lower figures

show the lower and upper sides more highly magnified. The specimen is

one in which the canals are unusually well seen.]

 

It may be well, however, to sum up the evidence as it has been

presented by Sir W. E. Logan, Dr. Carpenter, Dr. Hunt, and the author,

in a short and intelligible form; and I shall do so under a few brief

heads, with some explanatory remarks:--

 

1. The Lower Laurentian of Canada, a rock formation whose

distribution, age, and structure have been thoroughly worked out by

the Canadian Survey, is found to contain thick and widely distributed

beds of limestone, related to the other beds in the same way in which

limestones occur in the sediments of other geological formations. There

also occur in the same formation, graphite, iron ores, and metallic

sulphides, in such relations as to suggest the idea that the limestones

as well as these other minerals are of organic origin.

 

2. In the limestones are found laminated bodies of definite form and

structure, composed of calcite alternating with serpentine and other

minerals. The forms of these bodies suggested a resemblance to the

Silurian Stromatoporæ, and the different mineral substances associated

with the calcite in the production of similar forms, showed that these

were not accidental or concretionary.

 

3. On microscopic examination, it proved that the calcareous laminæ

of these forms were similar in structure to the shells of modern

and fossil Foraminifera, more especially those of the Rotaline and

Nummuline types, and that the finer structures, though usually filled

with serpentine and other hydrous silicates, were sometimes occupied

with calcite, pyroxene, or dolomite, showing that they must when recent

have been empty canals and tubes.

 

4. The mode of filling thus suggested for the chambers and tubes of

Eozoon, is precisely that which takes place in modern Foraminifera

filled with glauconite, and in Palæozoic crinoids and corals filled

with other hydrous silicates.

 

5. The type of growth and structure predicated of Eozoon from the

observed appearances, in its great size, its laminated and acervuline

forms, and in its canal system and tubulation, are not only in

conformity with those of other Foraminifera, but such as might be

expected in a very ancient form of that group.

 

6. Indications exist of other organic bodies in the limestones

containing Eozoon, and also of the Eozoon being preserved not only in

reefs but in drifted fragmental beds as in the case of modern corals.

 

7. Similar organic structures have been found in the Laurentian

limestones of Massachusetts and New York, and also in those of various

parts of Europe, and Dr. Gümbel has found an additional species in

rocks succeeding the Laurentian in age.

 

8. The manner in which the structures of Eozoon are affected by the

faulting, development of crystals, mineral veins, and other effects of

disturbance and metamorphism in the containing rocks, is precisely that

which might be expected on the supposition that it is of organic origin.

 

9. The exertions of several active and able opponents have failed to

show how, otherwise than by organic agency, such structures as those

of Eozoon can be formed, except on the supposition of pseudomorphism

and replacement, which must be regarded as chemically extravagant, and

which would equally impugn the validity of all fossils determined

by microscopic structure. In like manner all comparisons of these

structures with dendritic and other imitative forms have signally

failed, in the opinion of those best qualified to judge.

 

Another and perhaps simpler way of putting the case is the

following:--Only three general modes of accounting for the existence

of Eozoon have been proposed. The first is that of Professors King and

Rowney, who regard the chambers and canals filled with serpentine as

arising from the erosion or partial dissolving away of serpentine and

its replacement by calcite. The objections to this are conclusive.

It does not explain the nummuline wall, which has to be separately

accounted for by confounding it, contrary to the observed facts,

with the veins of fibrous serpentine which actually pass through

cracks in the fossil. Such replacement is in the highest degree

unlikely on chemical grounds, and there is no evidence of it in the

numerous serpentine grains, nodules, and bands in the Laurentian

limestones. On the other hand, the opposite replacement, that of

limestone by serpentine, seems to have occurred. The mechanical

difficulties in accounting for the delicate canals on this theory are

also insurmountable. Finally, it does not account for the specimens

preserved in pyroxene and other silicates, and in dolomite and calcite.

A second mode of accounting for the facts is that the Eozoon forms are

merely peculiar concretions. But this fails to account for their great

difference from the other serpentine concretions in the same beds, and

for their regularity of plan and the delicacy of their structure, and

also for minerals of different kinds entering into their composition,

and still presenting precisely the same forms and structures. The only

remaining theory is that of the filling of cavities by infiltration

with serpentine. This accords with the fact that such infiltration by

minerals akin to serpentine exists in fossils in later rocks. It also

accords with the known aqueous origin of the serpentine nodules and

bands, the veins of fibrous serpentine, and the other minerals found

filling the cavities of Eozoon. Even the pyroxene has been shown by

Hunt to exist in the Laurentian in veins of aqueous origin. The only

difficulty existing on this view is how a calcite skeleton with such

chambers, canals, and tubuli could be formed; and this is solved by the

discovery that all these facts correspond precisely with those to be

found in the shells of modern oceanic Foraminifera. The existence then

of Eozoon, its structure, and its relations to the containing rocks and

minerals being admitted, no rational explanation of its origin seems at

present possible other than that advocated in the preceding pages.

 

If the reader will now turn to Plate VIII., page 207, he will find some

interesting illustrations of several very important facts bearing on

the above arguments. Fig. 1 represents a portion of a very thin slice

of a specimen traversed by veins of fibrous serpentine or chrysotile,

and having the calcite of the walls more broken by cleavage planes

than usual. The portion selected shows a part of one of the chambers

filled with serpentine, which presents the usual curdled aspect

almost impossible to represent in a drawing (_s_). It is traversed

by a branching vein of chrysotile (_s_´), which, where cut precisely

parallel to its fibres, shows clear fine cross lines, indicating the

sides of its constituent prisms, and where the plane of section has

passed obliquely to its fibres, has a curiously stippled or frowsy

appearance. On either side of the serpentine band is the nummuline

or proper wall, showing under a low power a milky appearance, which,

with a higher power, becomes resolved into a tissue of the most

beautiful parallel threads, representing the filling of its tubuli.

Nothing can be more distinct than the appearances presented by this

wall and the chrysotile vein, under every variety of magnifying power

and illumination; and all who have had an opportunity of examining my

specimens have expressed astonishment that appearances so dissimilar

should have been confounded with each other. On the lower side two

indentations are seen in the proper wall (_c_). These are connected

with the openings into small subordinate chamberlets, one of which is

in part included in the thickness of the slice. At the upper and lower

parts of the figure are seen portions of the intermediate skeleton

traversed by canals, which in the lower part are very large, though

from the analogy of other specimens it is probable that they have in

their interstices minute canaliculi not visible in this slice. Fig.

2, from the same specimen, shows the termination of one of the canals

against the proper wall, its end expanding into a wide disc of sarcode

on the surface of the wall, as may be seen in similar structures in

modern Foraminifera. In this specimen the canals are beautifully smooth

and cylindrical, but they sometimes present a knotted or jointed

appearance, especially in specimens decalcified by acids, in which

perhaps some erosion has taken place. They are also occasionally

fringed with minute crystals, especially in those specimens in which

the calcite has been partially replaced with other minerals. Fig. 3

shows an example of faulting of the proper wall, an appearance not

infrequently observed; and it also shows a vein chrysotile crossing the

line of fault, and not itself affected by it--a clear evidence of its

posterior origin. Figs. 4 and 5 are examples of specimens having the

canals filled with dolomite, and showing extremely fine canals in the

interstices of the others: an appearance observed only in the thicker

parts of the skeleton, and when these are very well preserved. These

dolomitized portions require some precautions for their observation,

either in slices or decalcified specimens, but when properly managed

they show the structures in very great perfection. The specimen in fig.

5 is from an abnormally thick portion of intermediate skeleton, having

unusually thick canals, and referred to in a previous chapter.

 

One object which I have in view in thus minutely directing attention

to these illustrations, is to show the nature of the misapprehensions

which may occur in examining specimens of this kind, and at the same

time the certainty which may be attained when proper precautions are

taken. I may add that such structures as those referred to are best

seen in extremely thin slices, and that the observer must not expect

that every specimen will exhibit them equally well. It is only by

preparing and examining many specimens that the best results can be

obtained. It often happens that one specimen is required to show well

one part of the structures, and a different one to show another; and

previous to actual trial, it is not easy to say which portion of

the structures any particular fragment will show most clearly. This

renders it somewhat difficult to supply one's friends with specimens.

Really good slices can be prepared only from the best material and by

skilled manipulators; imperfect slices may only mislead; and rough

specimens may not be properly prepared by persons unaccustomed to the

work, or if so prepared may not turn out satisfactory, or may not be

skilfully examined. These difficulties, however, Eozoon shares with