424 GEOLOGY. 



gists, so far as it applies to Germany. They regard some of the sepa- 

 rate epochs of Geikie as stages of a single epoch, and would reduce the 

 number of glacial epochs to three, so far as their country is concerned. 1 



The deposits of several distinct glacial epochs have been recog- 

 nized also in the mountains south of the ice-sheet, especially in the 

 Alps. 2 



In other continents the glacial formations have been studied in 

 detail in but few places, but recent studies in Turkestan indicate that 

 the history of the glacial period in the Thian Shan Mountains was 

 complicated, five glacial epochs being recognized. 3 



The loess of Europe and Asia has already been referred to. The 

 eolian hypothesis of its origin seems to be gaining in favor, but other 

 opinions have been held, 4 and still find advocates. 



The Cause of the Glacial Period. 



Many hypotheses respecting the cause of the glacial period have 

 been offered, but none of them has, as yet, commanded the general 

 assent of glacial investigators. 



Almost all hypotheses appeal to a combination of agencies, but 

 each centers more or less on some one agency which gives character 

 to the hypothesis. Grouped by their characteristic agencies, they 

 fall mainly into three classes: (1) those which appeal to elevation, 

 the hypsometric hypotheses; (2) those which appeal to phenomena 

 outside the earth, or to the relations of the earth to other bodies, the 

 astronomic hypotheses, and (3) those which appeal to changes in the 

 constitution or movements of the air, the atmospheric hypotheses. 



Hypsometric Hypotheses. 



The hypothesis of elevation. 5 — From the fact that alpine glacia- 

 tion is a function of elevation, it was natural that one of the earliest 

 hypotheses should postulate the lifting of the glaciated regions to 

 the snow-line by a wide-reaching deformative movement. Auxiliary 



'Keilhack, Jour. Geol., Vol. Ill, pp. 113-125. 



2 Penck, Die Alpen im Eiszeitalter. 



3 Huntington, Explorations in Turkestan, Carnegie Institution. 



4 fekertchleyandKingsmill,Q. J. G. S., Vol. LI (1895), pp. 238-254. 



5 Dana, Manual of Geology, 4th ed., p. 970, and Upham, Am. Geol., Vol. VI, p. 327, 

 and Am. Jour. Sci., Vol. XII, p. 33. 



