PRECAMBRIAN DESERTS 183 



Lake. The Gaisa beds on Varanger Fjord, in Norway, reach the same 

 latitude, and the Scotch Torridonian about latitude 58°. 



It is hard to imagine red soils, drifting sands, and the hot winds of 

 deserts as existing in regions now tundra-covered and frigid; but this 

 seems to have been true in the more northern areas. 



Glacial Periods 



Thus far arid conditions only have been mentioned, but the best pre- 

 served land surfaces of the past are those sealed up unchangeably beneath 

 glacial deposits. It seems absurd to couple together deserts and glaciers, 

 so opposite to one another in every respect ; nevertheless in running down 

 the column of historical geology one finds these contradictory phenomena 

 closely linked together. In almost all the periods where aridity has been 

 proved there have been found also proofs of ice-action, the two seemingly 

 hostile conditions occurring either at the same time in different parts of 

 the world or one after the other in the same region. We live in the clos- 

 ing stages of a great Glacial period, extensive ice-sheets still surviving in 

 Greenland and the Arctic islands, as well as in Antarctica, and yet wide 

 deserts are found in all continents save Europe. 



More or less certain evidence of ice-action has been found in the Pleis- 

 tocene, the Eocene, the Cretaceous, the Triassic, the Permian, or Permo- 

 carboniferous, the Carboniferous, the Devonian, or possibly Upper Silu- 

 rian, perhaps the Cambrian, certainly the late Precambrian, and the 

 Lower Huronian. The list just given is closely parallel to that given for 

 the arid periods. 



Only four of these glacial times are of prime importance — those of the 

 Pleistocene, the Permocarboniferous, the late Precambrian, and the 

 Lower Huronian. 



Pleistocene Ice Age 



The Pleistocene ice age, from which the world is just emerging, unless 

 this happens to be an interglacial period, is so familiar that little need 

 be said of it. Boulder-clay, moraines, and deposits formed by glacial 

 waters occur over (3,000,000 square miles of the northern hemisphere; 

 smaller areas are found in the southern hemisphere, and Pleistocene mo- 

 raines reach thousands of feet below the present glaciers on high moun- 

 tains all over the world, even under the equator, showing that the cli- 

 mates of the whole world were affected. Beneath the glacial deposits in 

 many places there are characteristically smoothed and striated rock sur- 

 faces, though near the edges of the ancient ice-sheets there are thousands 

 of square miles where loose materials were not swept aAvay to bedrock. 



