1 86 THE HABITAT OF THE EURYPTERIDA 



an extended discussion on migration the reader is referred to chap- 

 ter V on that subject below, especially pp. 203-7. These illustra- 

 tions 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 that they are out of date.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 fol- 

 lowed 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 Fluviatij.e 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 sub- 

 ject to the subaerial 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 



