1 68 The American Geologist. September, i89» 
until the observations w'ere fully confirmed by the detailed^ 
descriptions and illustrations in the elaborated paper by Cham- 
berlin and Salisbury, published in 1886. The explanation is 
supposedly found to lie in the peculiar relation of ice move- 
ment through the basins, the ice being diverted from this area. 
Until recently this area was thought to be unique but in 1891 
professor Salisbury discovered another small driftless area in" 
western Illinois. 
Transportatioti of Drift.. — The precise ma,nner in which the 
ice carried its burden of rock debris has been in question. Mr. 
Uphamhas emphasized the amount of enelacial drift. He 
argues that the differential flow^of the ice induces ascending 
currents which lift the basal or subglacial drift into higher 
planes of the ice body. - On the other hand professor Cham- 
berlin claims that the continental ice mass never held any- con^^ 
siderable burden of englacial' drift except near the base; and 
he finds support for his position in the essentially basal charac- 
ter of the debris in' the Greenland glaciers. The existence of 
ascending currents in the ordinary flow of the ice sheet is not 
admitted by most students of glacial physics. The question is 
of some moment in relation to the manner of accumulation of 
■localized masses of debris, such as' drumlins, moraines and 
eskers. ■ 
As a philosophical discussion of the character, and work 
of a: local ice-body, the writings of W J McGee upon the 
glacial phenomena of northeastern Iowa deserve special men- 
tion. • ' 
GLACIAL PERIOD. 
Cause.- — The cause of the glacial period remains quite as 
much a mystery as it was in 1840. A large body of facts has 
been collected, but it points in different directions. Every 
person has entire liberty, of opinion; most glacialists have no 
opinion at all upon this subject. Up to 1875 the Lyellian hy- 
pothesis of land elevation as a cause of the cold climate and 
snow accumulation had the majority of adherents. In Amer- 
ica this was proposed by C: B'. Adams' in 1850, and wa^ 
adopted'by Daha in his presidential address of 1855; and was 
held by him during his life. It has been the consistent teaching 
of the Manual of Geology. Of preglacial elevation of northern 
land there is little doiibtj but many glaCialists find reason for 
