THE CAUSE OF THE GLACIAL PERIOD. 513 



cession, there seems to have been a renewal of elevation, as 

 shown by the height and slopes of the loess. 



Asia had no extensive ice-sheet like those of Europe and 

 North America, probably because a sufficient elevation was not 

 attained there until the Himalayas and Thibet were uplifted in 

 the Glacial period. Their southern latitude and the position 

 of Thibet and Mongolia in an arid and partly rainless belt, 

 which stretches thence west to the Sahara, forbade their glacia- 

 tion ; but from these recently uplifted Asiatic table-lands and 

 mountains the most extensive Quaternary deposits of the world 

 have been brought down by rivers and spread in the vast low 

 plains of Siberia, eastern China, and northern India, sloping 

 gently toward the sea, into which the finer part of this allu- 

 vium is carried. All the puzzling features of the Chinese loess 

 formation,* reaching to great elevations with such thickness 

 and slopes of its surface that it could not be so accumulated as 

 alluvium of flooded streams under the present conditions, seem 

 to be readily explained by referring its deposition to annual 

 floods from immense snow-melting, during the European Gla- 

 cial epochs, upon the gradually rising central part of the Asi- 

 atic Continent, which consists largely of easily erosible strata, 

 and had in preglacial time become extensively disintegrated by 

 weathering under a dry climate. 



With this reference of glaciation primarily to oscillations 

 of land, a new element of Quaternary history is introduced, 

 which seems to help much in accounting for peculiarities in 

 the areal distribution of identical or closely allied species of 

 animals and plants that have doubtless sprung from a common 

 source but have now become widely separated. Not only are 

 we able to follow Gray in his tracing the origin of the big trees 

 of California, of the species in the flora of the eastern United 

 States — which are the same with species of Japan, China, and 

 the Himalayan region, or are represented there by closely re- 

 lated forms, though unrepresented in Europe — and of mount- 



* Baron Richthofen, "Geological Magazine," II, vol. ix, 1882, pp. 293-305. 

 J. D. Whitney, " American Naturalist," vol. xi, pp. 705-713, December, 1877. 

 R. Pumpelly, " American Journal of Science," III, vol. xvii, pp. 133-144, Feb- 

 ruary, 1879. E. W. Hilgard, "American Journal of Science," III, vol. xviii, 

 pp. 106-112, August, 1879. 



