ENVIRONMENTAL CONDITIONS 399 



could, however, attain a free life, and meeting the physical conditions of 

 that environment became active swimming organisms. A retrogressive, 

 sluggish, bottom-living offshoot became armored, pressed out from the 

 locus of evolution, and because of that armor was the first group to give 

 fossil evidence of their existence. But, as ostracoderms, they emigrated 

 as scavengers, not as active rulers, and with the progress of evolution they 

 were extinguished without issue. Much of the Silurian was marked by 

 semi-arid to arid climatic conditions. It is probable that under this cli- 

 matic control the ganoids found their origin. In a recurrence of these 

 conditions in the time of deposition of the Old Eed Sandstone they spread 

 and finally dominate within the land waters over the selachians. The 

 beginning of the Old Red Sandstone formations in the late Silurian marks 

 a spread and diversification of the lands and their waters. The crust 

 movements involve changing environments, a stimulus to evolution. The 

 river and lake deposits at the same time permit the burial of organic 

 remains. The record thus becomes clear and inference gives place to 

 more definitely ascertainable fact. 



ENVIRONMENT OF FISHES OF THE OLD BED SANDSTONE 



A study of the Old Eed Sandstone of the Appalachian geosyncline has 

 been made by the writer in the field and of the somewhat similar British 

 deposits from the reports of the British geologists. The conclusion has 

 been reached from these studies that fluviatile deposition on the flood- 

 plains of inland basins was at that time an important mode of sedimenta- 

 tion. Seasons of flowing fresh waters over the regions of accumulation 

 alternated with seasons of exposure to the dry air. On the outer zones 

 subaerial delta plains passed into mud-bottomed shallow seas. In the 

 inner basins shallow lakes with shifting margins were existent at times. 

 At other times the lakes were reduced to pans of water, evaporating in 

 each dry season. The reduction in lake area would be due to a decrease 

 in rate of subsidence of the basin floor or in amount of rainfall, on the one 

 hand ; or, on the other, to an increase in sedimentation or evaporation. 



The great volume of the sediments which remain is only a fraction of 

 , the far greater volume which resulted from the destruction of Devonian 

 mountains. The area of Devonian uplands which supplied the waste 

 must have been large, and its debris was transported by rivers which 

 passed from the uplands out over the basin plains. The fauna of these 

 rivers would not have been greatly different in the regions of erosion from 

 what it was in those of deposition, but fossils could be buried and pre- 

 served only in the latter regions. 



The nature of the climate impresses itself on the sedimentary record, 

 XXIX — Bull. Geol. Soc. Am., Vol. 27, 1915 



