DRY LAND IN GEOLOGY COLEMAN. 263 



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 closing 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 

 Pleistocene, the Eocene, the Cretaceous, the Triassic, the Permian, or 

 Permocarboniferous, the Carboniferous, the Devonian, or possibly 

 Upper Silurian, perhaps the Cambrian, certainly the late pre-Cam- 

 brian, and the Lower Huronian. The list just given is closely par- 

 allel 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 pre-Cambrian, 

 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. Bowlder-clay, moraines, and deposits formed 

 by glacial waters occur over 6,000,000 square miles of the Northern 

 Hemisphere; smaller areas are found in the Southern Hemisphere, 

 and Pleistocene moraines reach thousands of feet below the present 

 glaciers on high mountains all over the world, even under the Equa- 

 tor, showing that the climates of the whole world were affected. Be- 

 neath the glacial deposits in many places there are characteristically 

 smoothed and striated rock surfaces, though near the edges of the 

 ancient ice sheets there are thousands of square miles where loose 

 materials were not swept away to bedrock. The central areas were 

 most effectively scoured, and in many places the rocks beneath, owing 

 to unequal hardness, have been shaped into roches moutonnees, 

 forming hills well rounded on the side from which the ice advanced. 

 Bowlder-clay is a highly specialized product of land ice; floating 

 ice, such as floes or bergs, is not known to produce it, the materials 

 dropped through the water when melting being necessarily more or 

 less stratified. The " soled bowlders " or " striated stones " from 

 bowlder-clay have special characters not caused by any other agency, 

 such as mudflows or torrential action. They are manufactured arti- 

 cles, easily recognized by one familiar with glacier work, and not to 

 be confounded with stones scratched or smoothed in other ways. 

 These familiar features are recalled because they serve as criteria 

 for the recognition of the ancient ^laciations to be mentioned later. 



