182 A. P. COLEMAN DRY LAND IN GEOLOGY 



York, Ohio, Ontario, and Manitoba ; and the succeeding Old Eed 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 Precambrian Deserts 



Desert characters have been ascribed to sandstones, perhaps belonging 

 to the earliest Cambrian, but more probably the uppermost Precambrian, 

 in many parts of the world. They include apparently the Keweenawan 

 and part of the Belt series in America, the Torridonian of Scotland, part 

 of the Gaisa beds of Norway, perhaps also the Sparagmite of Sweden and 

 the Jotnian of Finland. Whether the Matsap beds of Cape Colony and 

 some of the Kuddapah sandstones of India, described as shore deposits, or 

 the Vindhian sandstones and conglomerates should be included is uncer- 

 tain. 



If these are all of the same age and have been correctly interpreted as 

 arid deposits, this was the most severe and extensive period of desert con- 

 ditions known. In many places on the Canadian Shield the coarse red 

 sandstones, usually with some conglomerate at the base, may be seen rest- 

 ing on an Archean surface of granitoid gneiss or Keewatin schist or 

 Animikie slate, the original land surface of gently rounded hills and 

 shallow valleys belonging to an ancient peneplain. In some outcrops the 

 crumbling gneiss beneath, an old regolith, provides most of the materials 

 for the basal conglomerate. This is true at various points on the north 

 shore of Lake Superior and apparently also in Scotland, where the Tor- 

 ridonian rests on the Lewisian. The Lake Superior Keweenawan, though 

 much the best known, is on a small scale as compared with the areas of 

 sandstone of the same age farther north in Canada. The Athabasca sand- 

 stones of Tyrrell, those of Great Bear Lake and of central Labrador, not 

 to speak of smaller areas, indicate a very broad surface exposed to arid 

 conditions in North America. These red sandstones still occupy not less 

 than 50,000 square miles, and it is certain that much greater areas of 

 such relatively soft and easily attacked rocks have been destroyed in the 

 long dry-land periods of later times. 



It appears that in this desert period the arid districts were mainly in 

 the northern hemisphere and to the north of latitude 48° — that is, very 

 much farther north than the belt of deserts of the present northern 

 hemisphere. It is unknown, of course, to what extent Keweenawan rocks 

 are buried to the south of Lake Superior or of Scotland. The breadth of 

 the belt as known in North America is at least 20°, since rocks of this 

 ao-e reach nearly to 70° north latitude in the region north of Great Bear 



