It is also to be observed that independently of volcanic debris the reports of the Challenger expedition show that in the deep seas the decay of organic matter causes an alkaline condition of the sediments leading to the formation of alkaline silicates, while the presence of decaying volcanic dust furnishes the basis, whether of iron, alumina, or magnesia, necessary for the making up of glauconite. I have also suggested that the assimilation by Protozoa making calcareous skeletons, of the matter of Diatoms or humble plants having soluble silica in their organization or of silicious Protozoa, and sponge germs, must set free much soluble silica as a rejected or excrementitious matter which may contribute to the same result.
It is much more likely that the serpentine of the Laurentian limestones was produced in these ways than that it resulted from the hydration of magnesian minerals after the rock was consolidated. In the former case it would be in the most favourable conditions for mineralizing organisms as glauconites do in the modern seas. In the latter it would cause disturbances and changes of volume of which we have no evidence.
We thus find that the chemistry of the modern seas and that relating to the preservation of fossils of various ages by silicious infiltrations lends great probability to the belief that serpentine played this role in the oldest seas, though it would seem that dolomite was more suitable to the filling of the extremities of the minute tubes and their finer terminations.[35]
[35] I have shown also that in the limestone containing Eozoon we find layers holding concretions of serpentine alternating with others holding crystals of dolomite, as if there were at some times conditions favourable to the deposition of silicate of magnesia, and at others to that of the carbonate.
Fig. 43.—Stromatocerium rugosum, Hall, Ordovician.
(3) Our third question leads to the inquiry in what modern or ancient marine animals we can find structures akin to those of our supposed Laurentian fossil. The first analogy which suggested itself to Sir W. Logan, and a very natural one, was that to the so-called layer-corals (Figs. [43] to [45]) that abound in the Silurian, Ordovician, and Cambrian rocks, and which though undoubtedly fossil animals, have proved very difficult to interpret or to assign to any known group. At first vaguely associated with the true corals, they were subsequently regarded as probably of more simple character, and as gigantic Protozoa; and later strong reasons have been assigned for giving them an intermediate place, as allied to those curious communities of humble animals possessing simple stomachs and prehensile tentacles (Hydroids) which form some of the simpler corals (Millepores, etc.), and the crusts (Hydractiniæ) which cover dead shells and other bodies in the sea. When examined microscopically, however, they differ very much among themselves, and it may be that some of them were Hydroids and some Protozoa.
Fig. 44.—Structures of Stromatopora.
(a) Portion of oblique section, (b) Wall with pores, and coated with crystals of quartz, (c) Thickened portion of wall with canals, (d) Laminæ and pillars.
