Part 16

The late Dr. T. Sterry Hunt contributed to the original paper on Eozoon in the Journal of the Geological Society, a valuable essay on the mineralization of fossils by serpentine, glauconite, and allied hydrous silicates. This was in part reprinted in the notes appended to one of the chapters of "The Dawn of Life," and the subject was further discussed by Hunt in his invaluable work, "Chemical and Geological Essays," and more especially in the chapter on the "Origin of Crystalline Rocks," a chapter which every geologist deserving the name should study with care.

I give here some of the more important facts referred to by Hunt, and may add that subsequent microscopic studies have familiarized me with the occurrence of serpentine and other hydrous silicates as fillings of the cavities of fossils of various geological ages, insomuch that I have come to regard the occurrence of these rocks in association with fossiliferous limestones as among the best available means to enable us to ascertain the minute structures of shells, Foraminifera, corals, etc.

The following remarks and analyses further illustrate Hunt's views on the relations of these minerals, with some of the facts on which they are based:--

"In connection with the Eozoon it is interesting to examine more carefully into the nature of the matters which have been called glauconite or green-sand. These names have been given to substances of unlike composition, which, however, occur under similar conditions, and appear to be chemical deposits from water, filling cavities in minute fossils, or forming grains in sedimentary rocks of various ages. Although greenish in colour, and soft and earthy in texture, it will be seen that the various glauconites differ widely in composition. The variety best known, and commonly regarded as the type of the glauconites, is that found in the green-sand of Cretaceous age in New Jersey, and in the Tertiary of Alabama; the glauconite from the Lower Silurian rocks of the Upper Mississippi is identical with it in composition. Analysis shows these glauconites to be essentially hydrous silicates of protoxyd of iron, with more or less alumina, and small but variable quantities of magnesia, besides a notable amount of potash. This alkali is, however, sometimes wanting, as appears from the analysis of a green-sand from Kent, in England, by that careful chemist, the late Dr. Edward Turner, and in another examined by Berthier, from the calcaire grassier, near Paris, which is essentially a serpentine in composition, being a hydrous silicate of magnesia and protoxyd of iron. A comparison of these last two will show that the loganite, which fills the ancient Foraminifer of Burgess, is a silicate nearly related in composition.

I. Green-sand from the calcaire grossier, near Paris. Berthier (cited by Beudant, "Mineralogie," ii., 178).

II. Green-sand from Kent, England. Dr. Edward Turner (cited by Rogers, Final Report, Geol. N. Jersey, page 206).

III. Loganite from the Eozoon of Burgess.

IV. Green-sand, Lower Silurian; Red Bird, Minnesota.

V. Green-sand, Cretaceous, New Jersey.

VI. Green-sand, Lower Silurian, Orleans Island.

The last four analyses are by myself."

I. II. III. IV. V. VI. Silica 40·0 48·5 35·14 46·58 50·70 50·7 Protoxyd of iron 24·7 22·0 8·60 20·61 22·50 8·6 Magnesia 16·6 3·8 31·47 1·27 2·16 3·7 Lime 3·3 2·49 1·11 Alumina 1·7 17·0 10·15 11·45 8·03 19·8 Potash traces 6·96 5·80 8·2 Soda ·98 ·75 ·5 Water 12·6 7·0 14·64 9·66 8·95 8·5 ---- ---- ------ ----- ------ ----- 98·9 98·3 100·00 100·00 100·00 100·0

An eminent example is the Silurian limestone of Pole Hill, in New Brunswick, collected by the late Mr. Robb, of the Geological Survey, and referred to in the text. I cannot doubt that the silicate injecting Crinoids and other fossils in this limestone must have been introduced into these when still recent, and the same remark applies to the serpentine filling a coral at Lake Chebogamong, and fragments of corals at Melbourne, in Eastern Canada, and to the similar mineral filling fossils in a limestone from Llangwyllog, in Wales, and in that of Maxville, Ohio. Hunt regarded all these as coming essentially into the same category as regard to general composition and properties. His analysis of the minerals from Pole Hill and Llangwyllog is as follows:--

Pole Hill. Llangwyllog. Silica 38·93 35·32 Alumina 28·88 22·66 Protoxyd of iron 18·86 } 24·12 } Magnesia 4·25 } 6·96 } Potash 1·69 } 1·40 } Soda ·48 } 0·67 } Water 6·91 11·46 Insoluble, quartz ------ ----- 100·00 99·89

These minerals approach in composition to the jollyte of Von Kobell, from which they differ in containing a portion of alkalies, and only one half as much water. In these respects they agree 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; the Eozoon itself being there injected with a hydrous silicate which may be described as intermediate between glauconite and chlorite in composition."

In the Welsh specimen the silicate is of a deep green colour, except where oxidized, and though only 3 per cent, of the whole, is sufficient to give it an olive colour and slight serpentinous lustre. In the Pole Hill material, the silicate amounts to 5 per cent, of the whole, and is of a greyish colour. For some further particulars, see my Paper on "Fossils Mineralized with Silicates" (Journal Geological Society, February, 1879).

* * * * *

C. Affinities of Eozoon, etc., with more Modern Forms.

Dr. Carpenter, who in admirable papers, which I need not quote here, has illustrated in detail the structures of Eozoon, and shown its resemblance to modern forms, places Eozoon as a generalized type between the Nummuline and Rotaline groups of Foraminifera. It resembles the former in its fine and complicated tubulations, and some of the larger sessile forms of the latter in its habit of growth. More especially, this is near to that of the genera Carpenteria and Polytrema. In the former, more especially, there are a number of somewhat flattened calcareous cells with perforated walls, and built up in a conical form around a central pipe or funnel into which the apertures of the cells open. A specimen of Carpenteria, enlarged and having the walls of its cells thickened by a supplemental tubulated deposit like that of Calcarina, would approach very near to Eozoon.

W. B. Carpenter on Eozoon Canadense. Intellectual Observer, No. xl., p. 300, 1865. Supplemental notes on the structure and affinities of Eozoon Canadense, Quart. Journ. Geol. Soc., Lond. Vol. xxii., pp. 219-228, 1866. Notes on the structures and affinities of Eozoon Canadense. Canad. Nat., new ser., vol. ii., pp. 111-119, wood-cut, 1865. A reprint from Quart. Journ. Geol. Soc., Lond., 1865. Further observations on the structure and affinities of Eozoon Canadense. In a letter to the President. Proc. Roy. Soc., Lond., vol, xxv., pp. 503-508, 1867. New observations on Eozoon Canadense. Ann. and Mag. Nat. Hist., sen 4, vol. xiii., pp. 456-470, one plate, 1874. Final note on Eozoon Canadense. Ann. and Mag. Nat. Hist., ser. 4, vol. xiv., pp. 371-372, 1874. Remarks on Mr. H. J. Carter's letter to Prof. King on the structure of the so-called Eozoon Canadense. Ann. and Mag. Nat. Hist., ser. 4, vol. xiii., pp. 277-284, with two engravings, 1874.]

The question of the general relation of an organism like Eozoon to creatures known to us in the modern seas may be answered in either of two ways:--(1) Functionally or in relation to the position of such an animal in nature: or (2) Zoologically, or with reference to its affinities to other animals. With reference to the first consideration, the answer is plain. The geological function of Eozoon was that of a collector of calcareous matter from the surrounding waters, then probably very rich in calcium carbonate, and its role was the same with that of the Stromatoporæ and calcareous Sponges, smaller Foraminifera and Corals in latter times. The answer to the second aspect of the question is less easy. An ordinary observer would at once place Eozoon with the Stromatoporidæ or Layer-corals, which fill or even constitute whole beds of limestone in the Cambro-Silurian, Silurian and Devonian Periods. While, however, Eozoon has been claimed on the highest authority for the Rhizopods, the Stromatoporæ and their allies have been regarded as Sponges, or more recently as Hydroids allied to the Hydractiniæ and Millepores. I confess that I am not satisfied with these interpretations. I have in my collections large numbers of encrusting spinous forms, usually called Stromatoporæ, but which I have long set aside as probably Hydractiniæ. There are other forms with large vertical tubes which I have regarded as corals, but some Stromatoporæ seem to be different from either, and I am still disposed to regard many of them as Protozoa. Bearing in mind, however, that the Silurian is as remote from the Laurentian on the one hand as from the Tertiary on the other, we might be prepared to expect that if the Layer-corals of the Silurian are divisible into different groups, somewhat widely separated, and we have in the lower Palæozoic the peculiar type of Cryptozoon, we may be prepared to expect in the Laurentian much more generalized forms, less susceptible of classification in our modern systems. If, therefore, Eozoon were accessible to us in a living state, I should not be surprised to find that--while perhaps more akin to the calcareous-shelled Rhizopods than to any other modern group--it may have presented points of resemblance to Sponges or even to Hydroids, in its skeleton and mode of growth, and even in the arrangement of its soft parts.

Taking this view of its nature and relations, the genus and the Laurentian species may be characterized as follows:--

Genus Eozoon, Dawson.

Foraminiferal skeletons, with irregular and often confluent cells, arranged in concentric and horizontal laminæ, or sometimes piled in an acervuline manner. Septal orifices irregularly disposed. Proper wall finely tubulated. Intermediate skeleton with branching canals.

Eozoon Canadense, Dawson.

In inverted conical or rounded masses or thick encrusting sheets, frequently of large dimensions. Typical structure stromatoporoid, or with concentric calcareous walls, frequently uniting with each other, and separating flat chambers, more or less mammillated, and spreading into horizontal lobes and small chamberlets; chambers often confluent and crossed by irregular calcareous pillars connecting the opposite walls. Upper part often composed of acervuline chambers of rounded forms. Proper wall tubulated very finely. Intermediate skeleton largely developed, especially at the lower part, and traversed by large branching canals, often with smaller canals in their interstices. Lower laminæ and chambers often three millimetres in thickness. Upper laminæ and chambers one millimetre or less. Age Upper Laurentian and perhaps Huronian.

Var. minor.--Supplemental skeleton wanting, except near the base, and with very fine canals. Laminæ of sarcode much mammillated, thin, and separated by very thin walls. Probably a depauperated variety.

Var. acervulina.--In oval or rounded masses, wholly acervuline. Cells rounded; intermediate skeleton absent or much reduced; cell-walls tubulated. This may be a distinct species, but it closely resembles the acervuline parts of the ordinary form.