348 A. p. COLEMAN GLACtAL PERIODS AND GEOLOGICAL THEORIES 



dence is naturally harder to obtain as one goes backward in the geo- 

 logical records, but a late Paleozoic (Permo-Carboniferous) ice age is 

 generally accepted as proved for large parts of the world, and an early 

 Cambrian ice age seems to have left its record in several widely separated 

 regions. There is also strong evidence, as shown in a previous paper in 

 the Journal of Geology, of a Lower Huronian ice age whose effects are 

 found over hundreds of thousands of square miles. These four glacial 

 periods are the most important so far as known, though supposed proofs 

 of ice action have been found in the Cretaceous, the Lower Carboniferous, 

 the Lower Devonian, and at other horizons. In the latter cases the area 

 affected seems to have been only small, and the work may have been done 

 by local alpine glaciers rather than continental ice-sheets, such as left 

 their mark in the four periods mentioned above. It is probable that local 

 glaciers have existed on lofty mountains from the earliest ages, even when 

 the lower regions had a warm climate extending toward the poles ; but ice 

 ages imply a much more important refrigeration, reaching low groimd 

 in regions now having a temperate or even a tropical climate. 



The Pleistocene Glacial Period 



One naturally begins an account of ice ages with the last one, which 

 filled most of Pleistocene time and whose ice-sheets still linger in Arctic 

 and Antarctic regions and on high mountains to show us how the work 

 was done. 



Briefly summing up the extent of its work, one may say that 4,0()i),00r 

 square miles of North America were covered by ice-sheets, includirg the 

 whole of Canada except the Yukon territory and most of the northeast- 

 ern United States, with a southern limit of latitude 37° 30' in the Missis- 

 sippi valley. About half as much territory was ice-covered in nortliern 

 Europe, the southern boundary being about latitude 53°, and the great 

 moamtain ranges of Europe, especially the Alps, were heavily glaciated 

 south of the main sheet. 



In Asia there appear to have been no great continental ice-sheets, but 

 in almost all the mountain areas there were glaciers where none now 

 exist, or the glaciers were far more extensive than at present. Lebanon, 

 the Caucasus, the Himalayas, and the mountains of Manchuria all show 

 ice-scored surfaces and moraines, even reaching down to 4,500 feet above 

 the sea in latitude 26° in Bengal, and to 2,000 or 3,000 feet in the west- 

 ern Himalaya. 



In Africa the Atlas mountains bore glaciers, and on the lofty peaks 



