Though Eozoon was probably not the only animal of the Laurentian seas, yet it was in all likelihood the most conspicuous and important as a collector of calcareous matter, filling the same place afterwards occupied by the reef-building corals. Though probably less efficient than these as a constructor of solid limestones, from its less permanent and continuous growth, it formed wide floors and patches on the sea bottom, and when these were broken up, vast quantities of limestone were formed from their débris. It must also be borne in mind that Eozoon was not everywhere infiltrated with serpentine or other silicious minerals; quantities of its substance were merely filled with carbonate of lime, resembling the chamber wall so closely that it is nearly impossible to make out the difference, and thus is likely to pass altogether unobserved by collectors, and to baffle even the microscopist. Although, therefore, the layers which contain well characterised Eozoon are few and far between, there is reason to believe that in the composition of the limestones of the Laurentian it bore no small part, and as these limestones are some of them several hundred feet in thickness, and extend over vast areas, Eozoon may be supposed to have been as efficient a world-builder as the Stromatoporæ of the Silurian and Devonian, the Globigerinæ and their allies in the chalk, or the Nummulites and Miliolites in the Eocene. It is a remarkable illustration of the constancy of natural causes and of the persistence of animal types, that these humble Protozoans, which began to secrete calcareous matter in the Laurentian period, have been continuing their work in the ocean through all the geological ages, and are still busy in accumulating those chalky muds with which recent dredging operations in the deep sea have made us so familiar. (See Note appended.)
All this seems sufficiently reasonable, more especially since no mineralogist has yet succeeded in giving a feasible inorganic explanation of the combination of canals, laminæ, tubulation and varied mineral character existing in Eozoon. But then comes the strange fact of its apparent isolation without companions in highly crystalline rocks, and with apparently no immediate successors. This has staggered many, and it certainly, if taken thus baldly, seems in some degree improbable. Recent discoveries, however, are removing this reproach from Eozoon. The Laurentian rocks have yielded other varieties, or perhaps species of the genus, those which I have described as variety Acervulina, and variety Minor, and still another form, more like a Stromatopora, which I have provisionally named E. latior, from the breadth and uniformity of its plates.[56] There are also in the Laurentian limestone cylindrical bodies apparently originally tubular, and with the sides showing radiating markings in the manner of corals, or of the curious Cambrian Archæocyathus. Matthew, a most careful observer, has found in the Laurentian limestone of New Brunswick certain laminated bodies of cylindrical form, constituting great reefs in the limestone, and in the slates linear flat objects resembling Algæ or Graptolites, and spicular structures resembling those of sponges.[57] Britton has also described from the Laurentian limestone of New Jersey certain ribbon-like objects of graphite which he regards as vegetable, and names Archæophyton Newberryii.[58] Should these objects prove to be organic, Eozoon will no longer be alone. Besides this the peculiar bodies named Cryptozoum by Hall, and which are intermediate in structure between Eozoon and Loftusia, have now been found as low as the Lower Cambrian.[59] Barrois has also recently announced the discovery of forms which he regards as akin to the modern Radiolaria, creatures of a little higher grade than the Foraminifera, in the "Archæan" rocks of Brittany.[60] Thus Eozoon is no longer isolated, but has companions of the same great age with itself. The progress of discovery is also daily carrying the life of the Cambrian to lower beds, and thus nearer to the Laurentian. It is not unlikely that in a few years a pre-Cambrian fauna will force itself on the attention of the most sceptical geologists.
[56] Notes on Specimens of Eozoon, "Memoirs of Peter Redpath Museum," 1888.
[57] Bul. Nat. Hist. New Brunswick, No. IX., 1890.
[58] Annals N. Y. Academy of Science, 1888.
[59] Walcott, Lower Cambrian, 1892.
[60] Natural Science, Oct., 1892.
References:—"Life's Dawn on Earth," London, 1875. (Now out of print.) "Specimens of Eozoon Canadense in the Peter Red path Museum, Montreal," 1888. (This memoir contains reference to previous papers.)
Appended Notes.
1. Stromatoporæ. It has been usual of late to regard these as allies of the modern Millepores and Hydractineæ; but careful study of large series of specimens has convinced me that some species, notably the Stromatocerium of the Cambro-Silurian and the cryptozoum of the Cambrian, cannot be so referred. I hope to establish this in the future, if time permit.