Life's Dawn on Earth / Being the history of the oldest known fossil remains, and their relations to geological time and to the development of the animal kingdom

by Sir John William Dawson

"The oxygen ratios, as above calculated, are nearly as 3 : 2 : 1 : 1. This mineral approaches in composition to the jollyte of Von Kobell, from which it differs in containing a portion of alkalies, and only one half as much water. In these respects it agrees nearly with the silicate found by Robert Hoffman, at Raspenau, in Bohemia, where it occurs in thin layers alternating with picrosmine, and surrounding masses of Eozoon in the Laurentian limestones of that region;[AD] the Eozoon itself being there injected with a hydrous silicate which may be described as intermediate between glauconite and chlorite in composition. The mineral first mentioned is compared by Hoffman to fahlunite, to which jollyte is also related in physical characters as well as in composition. Under the names of fahlunite, gigantolite, pinite, etc., are included a great class of hydrous silicates, which from their imperfectly crystalline condition, have generally been regarded, like serpentine, as results of the alteration of other silicates. It is, however, difficult to admit that the silicate found in the condition described by Hoffman, and still more the present mineral, which injects the pores of palæozoic Crinoids, can be any other than an original deposition, allied in the mode of its formation, to the serpentine, pyroxene, and other minerals which have injected the Laurentian Eozoon, and the serpentine and glauconite, which in a similar manner fill Tertiary and recent shells."

[AD] Journ. für Prakt. Chemie, Bd. 106 (Erster Jahrgang, 1869), p. 356.

(C.) Various Minerals filling Cavities of Fossils in the Laurentian.

The following on this subject is from a memoir by Dr. Hunt in the Twenty-first Report of the Regents of the University of New York, 1874:—

"Recent investigations have shown that in some cases the dissemination of certain of these minerals through the crystalline limestones is connected with organic forms. The observations of Dr. Dawson and myself on the Eozoon Canadense showed that certain silicates, namely serpentine, pyroxene, and loganite, had been deposited in the cells and chambers left vacant by the disappearance of the animal matter from the calcareous skeleton of the foraminiferous organism; so that when this calcareous portion is removed by an acid there remains a coherent mass, which is a cast of the soft parts of the animal, in which, not only the chambers and connecting canals, but the minute tubuli and pores are represented by solid mineral silicates. It was shown that this process must have taken place immediately after the death of the animal, and must have depended on the deposition of these silicates from the waters of the ocean.

"The train of investigation thus opened up, has been pursued by Dr. Gümbel, Director of the Geological Survey of Bavaria, who, in a recent remarkable memoir presented to the Royal Society of that country, has detailed his results.

"Having first detected a fossil identical with the Canadian Eozoon (together with several other curious microscopic organic forms not yet observed in Canada), replaced by serpentine in a crystalline limestone from the primitive group of Bavaria, which he identified with the Laurentian system of this country, he next discovered a related organism, to which he has given the name of Eozoon Bavaricum. This occurs in a crystalline limestone belonging to a series of rocks more recent than the Laurentian, but older than the Primordial zone of the Lower Silurian, and designated by him the Hercynian clay slate series, which he conceives may represent the Cambrian system of Great Britain, and perhaps correspond to the Huronian series of Canada and the United States. The cast of the soft parts of this new fossil is, according to Gümbel, in part of serpentine, and in part of hornblende.

"His attention was next directed to the green hornblende (pargasite) which occurs in the crystalline limestone of Pargas in Finland, and remains when the carbonate of lime is dissolved as a coherent mass closely resembling that left by the irregular and acervuline forms of Eozoon. The calcite walls also sometimes show casts of tubuli…. A white mineral, probably scapolite was found to constitute some tubercles associated with the pargasite, and the two mineral species were in some cases united in the same rounded grain.

"Similar observations were made by him upon specimens of coccolite or green pyroxene, occurring in rounded and wrinkled grains in a Laurentian limestone from New York. These, according to Gümbel, present the same connecting cylinders and branching stems as the pargasite, and are by him supposed to have been moulded in the same manner…. Very beautiful evidences of the same organic structure consisting of the casts of tubuli and their ramifications, were also observed by Gümbel in a purely crystalline limestone, enclosing granules of chondrodite, hornblende, and garnet, from Boden in Saxony. Other specimens of limestone, both with and without serpentine and chondrodite, were examined without exhibiting any traces of these peculiar forms; and these negative results are justly deemed by Gümbel as going to prove that the structure of the others is really, like that of Eozoon, the result of the intervention of organic forms. Besides the minerals observed in the replacing substance of Eozoon in Canada, viz., serpentine, pyroxene, and loganite, Gümbel adds chondrodite, hornblende, scapolite, and probably also pyrallolite, quartz, iolite, and dichroite."

(D.) Glauconites.