346 J. BARRELL PLUVIATILE ORIGIN OP OLD RED SANDSTONE 



mations which make up this system have been known collectively since the 

 early days of geologic science as the Old Eed Sandstones. 



In Cornwall and southern Devon the Devonian is represented by slates, 

 grits, volcanic materials, and thick masses of limestone holding marine 

 fossils ; but in Ireland, Wales, Scotland, and the contiguous parts of Eng- 

 land the formations of equivalent age constitute the totally different 

 facies, the Old Eed Sandstones. These show their contrast not only in 

 lithologic character, but in their barrenness of marine fossils- The 

 Devonian age of these rocks was in fact only known at first because of 

 their intercalation between the formations of the Silurian below and 

 Carboniferous above, both identifiable by their marine fossils. 



What, then, were the geographic and climatic conditions which con- 

 trolled the nature of the Old Eed Sandstone deposits? The question is 

 raised to a higher degree of interest because the formations, though 

 barren of fossils of the sea, yet hold at certain horizons an abundant and 

 varied fauna of ostracoderms and fishes, the latter including forms which 

 lead toward the amphibians. 



The footprints of amphibians can be traced back to the close of the 

 Devonian period. It was in the Devonian that they doubtless rose from 

 air-breathing fishes. We must look, then, for our own ancestral tree 

 within certain families of these Old Eed Sandstone fossils. . Was it within 

 the physical conditions which determined the nature of the Old Eed Sand- 

 stone, or was it from the open sea that certain fishes grew to breath the 

 vivifying air, to crawl on the solid land, and inaugurate from such humble 

 beginnings that dynasty of terrestrial vertebrates which through all after 

 ages was to lead in the march of evolution and rule over the living things 

 of earth? It can be shown with high probability that it was from "the 

 faunas of the Old Eed Sandstone, molded in adaptation to the physical 

 and climatic conditions which surrounded them, that the amphibians 

 arose. 



Studies in evolution are commonly regarded as within the field only 

 of comparative anatomists, either students of fossils or of living forms ; 

 but the ancient life history of the earth embraces not only the organic 

 remains, as shown by the fossils, but a study of the environment which 

 surrounded them and to which their life activities made efficient response. 

 The analysis and evaluation of this environment is as important in the 

 complete mosaic of knowledge as is a minute description of the fossils of 

 successive faunas. Yet the student of fossils is not, by virtue of his 

 knowledge of fossils alone, qualified to interpret fully the life surroundings 

 of those former organisms. For this he must draw on his knowledge of 

 physical geology and the habitats of living forms. He must be able to 



