ARIDITY 181 



Arid Period of the Permian and Triassic 



Going back to Permian and Triassic times, much of the evidence has 

 been buried or destroyed ; yet it is certain that deserts extended widely in 

 many lands. Eed sandstones, arkoses, and shales with mud cracks and 

 footprints, beds of salt and gypsum, are reported from England, Germany, 

 Austria, and Russia in regions now well watered. In North America 

 there were the wide-spread red beds of the Rocky Mountain region and 

 the band of desert sandstones extending from Prince Edward Island 

 southwest to Virginia ; so that arid conditions covered far more of Europe 

 and North America than now. In India the Gondwana system includes 

 great- thicknesses of coarse sandstone with bands of conglomerate, sup- 

 posed to be of fluviatile origin, terrestrial deposits, but perhaps not of a 

 specially arid kind; 3 but no other references to Asiatic land conditions 

 have been found. I. C. White reports a thick series of massive red and 

 gray sandstones, probably of Triassic age, resting on Glossopteris beds 

 with coal seams in Brazil, but expresses no opinion as to the climate dur- 

 ing the deposition of these upper beds. The basal conglomerate under 

 the coal he thinks glacial. 4 Red beds of sandstone and conglomerate to 

 the thickness of 1,600 feet occur, according to Rogers, in the Karroo sys- 

 tem of South Africa, but he puts them probably above the Triassic. 5 

 Whether the 1,100 feet of Hawkesbury sandstones of the Triassic in New 

 South Wales, with their steep cross-bedding, bands of conglomerates, 

 worm tracks and sun cracks, imply an arid period in Australia is perhaps 

 uncertain, though they are undoubtedly continental deposits. 6 



It will be seen that land formations, often of a very arid kind, are 

 found in most of the continents in Permian or Triassic times. They seem 

 to occur rather later in the regions which endured the cold of the Permo- 

 carboniferous glaciation than in Europe and in our Western States, but 

 the correlation is not very certain. These "New Red" deserts following 

 on the heels of the severest ice age on record close the Paleozoic calami- 

 tously. It is not surprising that such extreme climatic changes put an 

 end to the lush growths of the coal swamps, so that only hardy plants 

 survived, and hastened the departure of the semi-aquatic amphibia, while 

 giving an impetus to the development of the reptiles as dry-land in- 

 habitants. 



There must have been very dry conditions during the Upper Silurian 

 (Salina) of America, as shown by the salt and gypsum beds of New 



3 Oldham : Geology of India, 2d edition, pp. 150-151. 



4 Brazilian coal fields, p. 31. 



c Geology of Cape Colony, p. 216. 



8 Geology of New South Wales, Suessmilch, pp. 158-160. 



