1 70 The American Geologist. Soptembcr, i89h 
time divisions adopted in a general way, some modifications 
liave been thought desirable. It is believed by many that 
the Champlain subsidence was far along, perhaps at its maxi- 
mum, before the ice sheet retreated. Indeed, Mr. Upham 
would make the Champlain only the closing part of the Glacial 
epoch. The "Recent" epoch is now regarded as essentially 
a part of the present epoch, and this view is strengthened by 
the demonstration, by Mr. Gilbert, that the northward differ- 
ential uplift of at least the Laurentian basin is still in progress. 
Interglacial Epochs. — Dana held firmly to the essential 
unity of the Glacial epoch, and in this he was largely followed, 
especially by the men who were trained upon the compara- 
.tively simple, homogeneous drift sheet of the eastern states. 
But in the broad areas of the west the drift is not so simple. 
Early in the study of the region buried soils, peat beds and 
other evidences of deglaciation were found intercalated in the 
drift, and were noted by Whittlesby. Newberry. Orton, N. H. 
Winchell and others. In 1873 Newberry regarded the "For- 
est Bed" as marking a distinct deglacial condition in the drift 
period. In 1878 McGee described vegetal deposits in the 
drift of northeastern Iowa of such depth and areal extent as 
to indicate an interglacial epoch, and he discriminated a later 
from an earlier till. Subsequently Chamberlin accepted these 
conclusions, and in 1882 he discovered the greater southern 
extension of the older drift and noted the evidences of a long 
intervat between. This idea of the duality or multiplicity of 
the glacial epochs met with long-continued opposition, but 
was found by the glacialists of the west a good working hypo- 
thesis, and evidences in its favor have multiplied. 
In 1889 Chamberlin regarded the fringe in Pennsylvania 
as the unburied edge of the older drift, and in 189 1 Salisbury 
described the extra-morainic drift of New Jersey and Penn- 
sylvania as the weathered and eroded border of a drift sheet 
far older than the moraine-bordered till. This was the first 
recognition in the eastern states of multiple drift phenomena, 
comparable to that in the western states. 
The study was given definiteness and placed on a working 
basis by the admirable article of professor Salisbury, in 1893, 
on the "Criteria for recognition of distinct glacial epochs." 
The application of these criteria by different workers, particu- 
