DRY LAND IN GEOLOGY COLEMAN. 261 



the Gondwana system includes great thicknesses of coarse sandstone 

 with bands of conglomerate, supposed to be of fluviatile origin, ter- 

 restrial deposits, but perhaps not of a specially arid kind ; ^ but no 

 other references to Asiatic land conditions have been found. I. C. 

 White reports a thick series of massive red and gray sandstones, prob- 

 ably of Triassic age, resting on Glossopteris beds with coal seams in 

 Brazil, but expresses no opinion as to the climate during the de- 

 position of these upper beds. The basal conglomerate under the 

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

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

 system of South Africa, but he puts them probably above the Tri- 

 assic.^ 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 con- 

 tinental deposits.* 



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 endure the cold of 

 the Permocarboniferous glaciation than in Europe and in our West- 

 ern States, but the correlation is not very certain. These " Ngav Red " 

 deserts following on the heels of the severest ice age on record close 

 the Paleozoic calamitously. 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 semiaquatic amphibia, while giving an impetus to the develop- 

 ment of the reptiles as dry-land inhabitants. 



There must have been very dry conditions during the Upper Silu- 

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

 New York, Ohio, Ontario, and Manitoba ; and the succeeding Old 

 Red beds of Scotland and other European countries suggest a similar 

 climate, but I have not found evidence of arid conditions on a wide 

 enough scale to make it desirable to discuss them here. 



LATE PRE-CAMBRIAN DESERTS. 



Desert characters have been ascribed to sandstones, perhaps be- 

 longing to the earliest Cambrian, but more probably the uppermost 

 pre-Cambrian, in many parts of the world. They include apparently 

 the Keweenawan and part of the Belt series in America, the Torri- 

 donian of Scotland, part of the Gaisa beds of Norway, perhaps also 

 the Sparagmite of Sweden and the Jotnian of Finland. Whether 



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



= Brazilian coal fields, p. 31. 



^ Geology of Cape Colony, p. 216. 



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



73839°— SM 1916 18 



