Page:The Habitat of the Eurypterida.djvu/185

Rh time says: "There is evidence that, during the time when the Orcadian Old Red was in course of being deposited, normal pluvial conditions obtained for a time. The deposition of ferric oxide in the old area of inland drainage ceased, chiefly in consequence of the large quantities of vegetable matter which were swept into the old lakes. This latter, in its turn, decomposed the solutions of sulphate of lime, and liberated the calcareous matter, which in a state of diffusion, or aggregated into nodules, now forms so conspicuous an element in the Orcadian Rocks. Furthermore, the sulphate of lime, in its turn, converted the vegetable matter into the bituminoids, which, in a diffused form, permeated—one might almost say saturated—so much of the Caithness Flagstones. I hold, therefore, that the exceptional durability of the Caithness flagstones, which of course is due to the large percentage of bituminous matter they contain, is due to the fact that conditions of inland drainage, one of the phases of desert conditions, prevailed where these occur during the Devonian Period" (80, 220).

. From data of the type just given, three theories have been evolved, each based upon practically the same observations in the field, but each involving very different interpretations. The oldest and most widely accepted explanation for the Old Red sandstone is that it is a series of lake deposits; the second theory, which quite rightly has never received very much attention, is that of marine deposition; the newest hypothesis is that the Old Red is dominantly of fluviatile origin and that the deposits were not laid down in any permanent body of standing water, either marine or fresh, but largely on the dry land as torrential and flood-plain deposits or in evanescent playas. I shall briefly consider the first two theories and the objections thereto, and shall then give the third and some of the evidence favoring it. All geologists are agreed that the sediments are clastic, that they were not deposited in the deep sea, that they are land-derived and river-transported; the only point of difference that has arisen is in regard to the locus of deposition.

Deposition in Lakes. This theory has been most fully expounded by Geikie and has been generally accepted in the form in which he gave it. For the British area he recognized five lakes on the basis of the present outcrops, considering that the heavy conglomerates marked the rocky lake shores of Devonic time, while finer deposits pointed out the central portions of the lakes. The presence of