Prof. Marsh has well explained another phase of the influence of hard bodies in producing partial slickensides, in his paper on Stylolites, read before the American Association in 1867, and the application of the combined forces of concretionary action and slickensiding to the production of the cone-in-cone concretions, which occur in the coal-formation and as low as the Primordial. I have figured a very perfect and beautiful form of this kind from the coal-formation of Nova Scotia, which is described in “Acadian Geology”[AA] ([Fig. 12]).
I have referred to these facts here because they are relatively more important in that older period, which may be named the age of Algæ, and because their settlement now will enable us to dispense with discussions of this kind further on. The able memoirs of Nathorst and Williamson should be studied by those who desire further information.
[AA] Appendix, p. 676, edition of 1878.
Fig. 12.—Cone-in-cone concretion (Carboniferous, Nova Scotia), illustrating pretended Algæ.
But it may be asked, “Are there no real examples of fossil Algæ?” I believe there are many such, but the difficulty is to distinguish them. Confining ourselves to the older rocks, the following may be noted:
The genus Buthotrephis of Hall, which is characterised as having stems, sub-cylindric or compressed, with numerous branches, which are divaricating and sometimes leaf-like, contains some true Algæ. Hall’s B. gracilis, from the Siluro-Cambrian, is one of these. Similar plants, referred to the same species, occur in the Clinton and Niagara formations, and a beautiful species, collected by Col. Grant, of Hamilton, and now in the McGill College collection, represents a broader and more frondose type of distinctly carbonaceous character. It may be described as follows:
Buthotrephis Grantii, S. N. ([Fig. 13]).—Stems and fronds smooth and slightly striate longitudinally, with curved and interrupted striæ. Stem thick, bifurcating, the divisions terminating in irregularly pinnate fronds, apparently truncate at the extremities. The quantity of carbonaceous matter present would indicate thick, though perhaps flattened, stems and dense fleshy fronds.