THE CAUSE OF THE GLACIAL PERIOD. 511 



along the whole complex east to northwest and west mountain 

 system of Oceania, Asia, Europe, and Northern Africa, from 

 New Guinea, the Sunda Islands, Anam, and Siam, to the Cau- 

 casus, Carpathians, Balkans. Apennines, Alps, Pyrenees, and 

 Atlas Mountains, stretching quite across the eastern hemi- 

 sphere ; but the greater part of the relief from the previously 

 existing deformation of the earth was doubtless along the cen- 

 tral part of the belt, in the colossal Himalayan range. In 

 like manner the North American Cordilleras and the Andes, 

 reaching in one continuous mountain system from the Arctic 

 Circle to Cape Horn, have experienced within the same period 

 great disturbances, as already noted, similar to those of the 

 mountains of Southern Europe and the adjacent part of Africa, 

 With this American orographic belt is also probably to be 

 associated the mountain system, consisting largely of volcanoes 

 now active, which forms the Aleutian Islands, Kamtchatka. 

 the Kurile Islands, Japan, Formosa, the Philippines, Borneo, 

 and Celebes, lying nearly in the same great circle with the 

 Andes and Rocky Mountains, and with them continuous in 

 an arc of about two hundred and forty degrees. Along two 

 lines transverse to each other, one having an extent of half 

 and the other of two thirds of the earth's circumference, the 

 great lateral pressures of the earth's crust, which probably 

 caused the elevation and glaciation of extensive areas during 

 the Quaternary period, have been relieved by plication, faults, 

 and uplifts, in the processes of the formation of mountain- 

 ranges.* 



Combined with oscillations of the earth's crust, which are 

 here regarded as the primary cause of the growth and decline 

 of ice -sheets, many other concomitant conditions, notably 

 changes in aerial and oceanic currents, and the earth's cycles of 

 twenty-one thousand years through precession and nutation, 

 enter into the complex causation of recurrent glacial and inter- 

 glacial epochs, and serve to intensify or to mitigate the severity 

 of the glaciation due to elevation. The influences of these 

 conditions would be nearly the same that are claimed for them 



* See Prestwich'p " Geology," vol. i, chap, xvii, treating of the relative ages 

 of the principal mountain-ranges of the world. 



