398 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



are oxidized. They are, in addition, leached of soluble matters by rain 

 and ground waters. In the low-lying portions permanent waters may 

 exist, giving rise to swamp and lake deposits ; but where the dry season is 

 prolonged, these permanent waters shrink in area. Where, on the con- 

 trary, there is no marked dry season, the greater part of the floodplain 

 may be kept saturated with water. This saturation, by preventing atmos- 

 pheric oxidation, will lead to the accumulation of shales which are dark 

 with carbon, or may even give rise to wide-spread coal formations. 



In climates with marked dry seasons, on the contrar}^, the floodplain 

 deposits are yellow, brown, or red; but, on consolidation into rock, the 

 color, because of the partial elimination of the water from the ferric 

 oxide, becomes more customarily brown or red. The deoxidized bands, 

 which may be interstratified, will turn to olive green. The Silurian and 

 Devonian continental deposits are commonly such brown and red forma- 

 tions, with minor amounts which are green or gray in color. Such for- 

 mations are peculiarly unfavorable for the preservation of organic re- 

 mains, for their high oxidation implies a maximum opportunity for the 

 destruction of organisms at the time of burial. 



The similar red shales and sandstones of Triassic age in Connecticut 

 preserve innumerable footprint impressions, testifying to an abundant 

 life ; yet fossils are extremely rare, and the skeletal forms of but few of 

 the very many species of vertebrates which made the footprints are 

 known. Even the greater of the dinosaurs which inhabited this region 

 left no record of their existence save in their gigantic birdlike tracks. 

 Fishes which must have lived freely in the flowing waters are preserved 

 only in the rare bands of black shale. 



This evidence points to the conclusion that, even if the fishes were an 

 abundant fauna in the continental deposits of the Silurian, they would 

 not be preserved in them as fossils save under the rarest of circumstances, 

 as impressions of scales, plates, or spines in variegated shales or fine- 

 grained sandstones. Such fossils are found in the Devonian, but in the 

 Silurian the spread of fishes was not yet so wide and the aridity was at 

 times more intense. The nature of the geologic record is, then, not in 

 disagreement with the hypothesis that the central habitat of fishes in the 

 Silurian may have been the fresh waters. The absence of fossils from 

 the river deposits and their rare preservation in the marginal waters of 

 the sea does not prove, on the other hand, that fishes were marine. The 

 case must be judged by other lines of evidence. 



Going back of the Silurian to the Ordovician, the absence of fresh- 

 water fossils is judged to be owing chiefly to physiographic rather than to 

 -climatic causes. The climate was such at times as to give rise to marine 



