Nez 
AN ATTEMPT TO FRAME A WORKING HYPOTHESIS 
ORI Ea CAUSE OR GLACIAL, RERIODS ONAN 
ALPVMOSEHERIC- BASIS. 
(Continued) 
ro: 
LOCALIZATION OF GLACIATION. 
THE problem of localization is in some sense independent of 
the fundamental hypothesis offered in this paper, and the sug- 
gestions which follow may be accepted or rejected without 
carrying necessarily an approval or disapproval of the main 
hypothesis. 
The remarkable distribution of the great ice-sheets —The chief 
centers of the Pleistocene ice-sheet lay on the north-northeastern 
plains of North America, and, on the northwestern quarter of 
Europe. On the northwestern Cordilleras there was also a nota- 
ble center, though it does not appear to have equaled the others 
in rank.‘ The north-northeastern American centers are properly 
regarded as chief because the spread of the ice-sheets from them 
t This statement is perhaps open to some question. It is quite certain from field 
observations that the glaciation of the Cordilleran plateau in the United States and 
in British Columbia as far north at least as 51° Lat. was much feebler than that of the 
Mississippi basin at corresponding latitudes and much lower present altitudes. It 
seems also clear that the ice of the north Cordilleras did not creep out upon the plains. 
to an extent at all comparable to the spread of the Scandinavian ice-sheet upon the 
plains of Europe. Considered from these points of view, and they seem to be the 
important ones, the statement can scarcely be questioned seriously. The evidences 
of glaciation on the mountainous border facing the Pacific from 48° northward arts, 
however, quite impressive. They find their climax perhaps in the 4000 or 5000 feet fof 
glacial! débris which Russell reports in the foothills of the St. Elias range. In v 4ew 
of this it may perhaps be insisted that the glaciation of this region was ex bep- 
tionally concentrated on the Pacific border, because of the abrupt rise of the sur ‘face 
and the height of the mountains, and that the glacial discharge toward the * Pacific 
was also exceptionally effective because the high gradient; so that, tak dng this 
intensification into account, the sum total of ice formation and ice action in ‘his region 
may not have been so much inferior to that of the European area as the /surface dis- 
tribution might seem to imply. a 
751 
