Glacial Geolog}> in America. — FaircJiild. 169 
thinking" that during the latter part of the glacial period the 
land was lower than at present, and certainly. wa& so. at the 
close. 
Croll's "Climate and Time" presented a plausible as- 
tronomical hypothesis,- the concurrence of variable elements in 
the relation of the earth to the sun affecting alternately the 
north and south polar regions. For a time this won large as- 
sent, but objections soon multiplied. The secular periods of 
glaciation apparently required by this hypothesis in preceding 
geologic time are quite lacking, although glacial drift deposits 
•occur as far back as. the Permian. The most serious objec- 
tion is the absence of glaciation over vast areas of arctic lands, 
as Alaska and Siberia. Eventhe physical competency of the 
astronomic changes has been questioned, and the hypothesis 
has fallen into, neglect as the tendency of late is to concentrate 
attention on phenomena and more immediate inferences. 
\ change in the axis of the earth is a suggestion that finds 
few supporters. A hypothesis of. atmospheric change,, a varia- 
tion in the amount of the carbon-dioxide content of the at- 
mosphere, has recently received the support of professor 
Chamberlin. 
The opinion of the majority of geologists . may probably 
be fairly stated by saying that the various elements affecting 
climate, geographic, atmospheric and astronomic, are thought 
to be so nicely balanced, that a comparatively slight change or 
-maladjustment may produce serious climatic effects.. 
Time Divisions.— Vxoit.^'sox Dana in 1855 proposed the 
tripartite division of the glacial period, correlating with land 
oscillation, into (i) the Glacial epoch with high elevation of 
land; (2) the Laurentian (Champlain), wnth depression of the 
land; and (3) a transition epoch, called the Terrace, during 
which the land rose to its present level and the river terraces 
were cut by the enlivened streams. This scheme was elabor- 
ated in the first edition of his Manual, in 1862, and adhered 
to with slight changes in all subsequent editions. In the 1879 
edition he gave the Terrace epoch a formal place, changing 
<the name in the last edition to "Recent." The name "Cham- 
plain," originally given to the marine clays by Edward Hitch- 
cock, was substituted for "Laurentian" in the edition of 1867. 
While thLs terminology has been generall\- used and the 
