ENVIRONMENTAL CONDITIONS 401 



stone groups which, yield the fishes. In the Upper Old Eed of Dura Den 

 the fossils are so crowded as to imply repeated wholesale destructions of 

 life. In contrast to the abundant and well preserved layers of fishes in 

 the dark-colored flags, the red sandstones show only occasional teeth, 

 scales, and spines. 



The piscine fauna of the Devonian lakes was doubtless as closely con- 

 nected with that of the rivers as are present lacustrine to present fluviatile 

 faunas. A greater difference would have separated the faunas of the 

 fresh and salt waters. It must be concluded, therefore, that the fishes 

 lived in the rivers as well as the lakes; but their fossils are well and 

 abundantly preserved only where the shallow lakes were occasionally ren- 

 dered unfit for life. The most obvious cause of such unfitness is found 

 in unusual seasons of dryness, reducing the shallow lakes to foul and 

 crowded pools. 



The Old Eed Sandstone formations of Great Britain were largely river 

 deposits on the subaerial surfaces of basins inclosed by growing mountain 

 ranges. The early Tertiary basins of the Cordillera in the western 

 United States give a fairly close analogy. In the eastern United States 

 the Devonian record is of a different nature. There is shown, especially 

 in New York and Pennsylvania, a continuous marine record of offshore 

 limestones, mudstones, and sandstones. Only in the Upper Devonian do 

 the terrestrial conditions reach broadly westward, as the Catskill delta, 

 across these States. In the offshore sands and muds of the Devonian is 

 a physical record deposited under more uniform physiographic conditions 

 than was the case with the British deposits. These sediments in the 

 Lower Devonian are largely limestones and fine-grained limy sandstones ; 

 in the Middle Devonian (Hamilton) they are thick deposits of muds and 

 sands colored dark with carbon. In the Upper Devonian marine deposits 

 (Chemung) carbon is wanting, and the colors are gray or olive green. 

 This marine record seems to show an increasing aridity from Middle to 

 Upper Devonian time, making the oxidation of sediments more complete. 

 The marginal marine deposits are not red, however, nor do they hold 

 gypsum or salt; so that here again is the evidence that the climate, unlike 

 that of some other times, was not highly arid, but was rather semi-arid 

 in its nature. Nevertheless it shows a climatic stress acting on the en- 

 vironment of fresh-water fishes and primitive amphibians, which increased 

 in the Upper Devonian. 



FAUNAL CHANGES IN THE OLD BED SANDSTONE 



The numbers of species and genera which occur and their distribution 

 under the different orders and subclasses have been tabulated for the 



