400 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



but impresses itself even more on the flora and fauna. The climate of 

 the Old Bed Sandstone basins is seen to have been of the dominant con- 

 tinental nature — semi-arid. This is a type of climate which needs to be 

 clearly visualized and separated from truly arid climates, which constitute 

 one extreme of the climatic gamut, and dominantly humid climates, which 

 mark the other limit. In a semi-arid climate the rainfall is seasonal, but 

 during the season of rains is sufficient in amount for the growth of grasses 

 or of other herbaceous vegetation. The growth of vegetation supports 

 abundant animal life. The great grazing lands of the United States, of 

 South Africa, and of Russia are illustrations. During the season of rains 

 the rivers are capable of carrying abundant waste, but during the season 

 of dryness the surface waters more or less completely disappear and the 

 vegetation withers. Sun, wind, and possibly frost in this dry season play 

 their parts in mechanical rock destruction and give some resemblances in 

 sedimentation to the character of desert deposits. This character of cli- 

 mate is, however, as stated, distinctly different in its geological and bio- 

 logical effects from either desert climates or humid climates. In deserts 

 vegetation is scanty and the chief transporting agent is wind. In humid 

 climates a forest cover can develop, giving deeper soils and a greater ratio 

 of chemical decay. 



There are biological indications of a generally warm temperature dur- 

 ing the Devonian period. Coral reefs grew in many places in the shallow 

 seas of Middle Devonian time and even in high latitudes. The fossils of 

 trees which have been preserved show no growth rings. They apparently 

 grew where permanent ground water was available for their roots, and 

 were able, therefore, in the absence of a winter, to grow throughout the 

 3"ear. 



A warm, semi-arid climate in the northern hemisphere was the mean, 

 but about this there must, in the long course of the Devonian, have been 

 many fluctuations. During the more uniformly wet epochs the rivers 

 would have been purer streams of more continuous flow, the shallow lakes 

 less subject to seasonal lowering of their water level by evaporation. 

 During the drier epochs the waters suitable for fishes would have been 

 reduced in area, even at the times of rains, and subject to great annual 

 diminution during the seasons of dryness. The severe restrictional effects 

 which such oscillations about a critical mean would have on the life of 

 fresh -water fishes is obvious. 



The fish fossils are especially abundant in certain horizons of the Old 

 Red Sandstone of Scotland. More or less perfect specimens are found in 

 flaggy, bluish, calcareous sandstones, or in gray, calcareous shales. A 

 bituminous cement is abundant in many cases. In the Middle Old Red 

 of the Orcadian basin mud-cracked layers occur repeatedly in the flag- 



