HYPOTHESIS OF CAUSE OF GLACIAL PERIODS 767 
southeast, and is constructively connected in that direction with 
the 30° belt, which develops a strong anticyclonic node near the 
California coast, beyond which it connects with the main tract 
which crosses the United States and the Atlantic near its normal 
position. Between this connection across the Pacific and the 
main loop before traced lies the Aleutian depression. 
The high tracts of the northern hemisphere therefore form 
(1) a great loop embracing the great north Atlantic depression 
and, (2) incidentally, as it were, a minor loop embracing the 
north Pacific depression. The great loop is not conceived as a 
current, much less a whirl, but as a tract along which the upper 
atmosphere habitually descends with outflowing tendencies. 
The enclosed area of low pressure is a tract over which the 
lower atmosphere habitually ascends with inflowing tendencies. 
Now the main loop and the main enclosed eddy he on one 
side of the northern hemisphere and embrace the chief area of 
Pleistocene glaciation. This main loop and its enclosed eddy 
embrace about three fourths of the hemisphere north of 30° N. 
Lat. In the area left vacant, so to speak, the secondary Aleutian 
eddy is developed and covers the main area of the Cordilleran 
Pleistocene glaciation. 
There can be no question that this peculiar configuration is 
due to physiographic influences, particularly the oblique attitude 
and peculiar oceanic circulation of the Atlantic, and the similar 
obliquity of the main body of the eastern continent. To realize 
the full force of this obliquity one should trace on a globe the 
great axis of the eastern continent from Cape Verde on the pro- 
truding portion of Africa to Cape East at the extremity of Asia, 
an immense spiral, and then, starting from the coast of South 
America near the lesser Antilles, trace the axis of the North 
Atlantic to the heart of the Polar sea ina similar great spiral, 
essentially parallel to the former. With these primary influences 
many secondary ones are joined, some acting concordantly to 
intensify the obliquity of the circulation, and others acting dis- 
cordantly and tending to destroy its symmetry, and modify its 
configuration. 
