an extended discussion on migration the reader is referred to chapter V on that subject below, especially pp. 203–7. These illustrations will suffice to show that fresh-water forms can often migrate for several thousand miles, and that through river distribution even the same species may occur in regions widely separated. It may here be remarked that distance is of less significance than time available for migration (see below, pp. 208 et seq.).
Summary. The objections to the marine and lacustrine theories of deposition for the Old Red may be reduced to the single criticism that they are out of date. The theories were helpful attempts toward the solution of one of the big problems in stratigraphy, but in their formulation and working out, their authors naturally followed the ideas which were accepted as correct twenty years ago; that some of these should have been found to need revision is only an evidence of the progress of science. The study of sedimentation is a branch of geology which is even yet not receiving the attention due it, but, nevertheless, the students of lithogenesis are steadily increasing, and there is more being said and written today about the work of the wind and of rivers in the geological past than there was a dozen years ago.
Theory of Fluviatile Deposition. The conditions up to the beginning of Old Red sandstone time have already been outlined and it was shown that there was a progressive retreat of the sea to the south, leaving all of Scotland and most of England a region of dry land subject to the subaërial forces of denudation, the greatest of which are the winds and the rivers. The rivers cutting down into the newly elevated continent carried great quantities of detritus toward the sea. But these were not the rivers of a pluvial climate. They were rather the torrents which carried off the waters from occasional heavy rains such as occur in semi-arid regions. That the climate must have been relatively dry is indicated by the thickness and great areal extent of the Old Red Sandstone, for, as was explained, these deposits must have been thoroughly oxidized at the time of their deposition in order that they might be potentially red. In post-Devonic time, either by age, heat or pressure, those oxidized deposits became red through dehydration. The climate, then, was semi-arid and the rivers of the nature of torrents which could transport vast quantities of material, but which would in most cases drop that material before reaching the sea. This would be brought about because the streams would soon lose their supply of water, for the rains were only periodic
