560 CEXOZOIC ERA— AGE OF MAMMALS. 



country at the base of the Alps, is attributed by Sacco to rain-wash of 

 bare soil recently left by the retreating ice.* 



It is probable, therefore, that several kinds of deposit, having a 

 superficial resemblance, have been confounded under the common term 

 of Loess, and that more observation is necessary to clear up the subject. 



«v7j III. Terrace Epoch. 



At the end of the epoch of subsidence, when the condition of sea 

 and lakes and rivers was what we have described, there commenced a 

 movement again in an opposite direction, by which the lands were slowly 

 brought upward to their present condition — a condition, however, far 

 less elevated than during the Glacial epoch. 



Evidences. — 1. Sea. — The re-elevation was not perfectly steady and 

 uniform, but stopped, from time to time, sufficiently long for the sea 

 to make distinct beaches. Below the highest beach, which marks the 

 maximum depression of the Champlain epoch, and which has already 

 been described, several other beaches are traceable, which evidently 

 mark the successive steps of re-elevation. 



2. Lakes. — Also, the re-elevation of the land would bring down the 

 level of the lakes, partly by change of climate diminishing the water- 

 supply, and partly by increasing the slope, and thereby increasing the 

 erosive power, of the discharge-rivers, and thus draining off the lake- 

 waters. This is well shown on the Canadian lakes, where, in addition 

 to the highest terrace, already mentioned, which marks the highest 

 flood-level of the Champlain epoch, are found several lower terraces, 

 which mark the successive stages of the subsequent depression of the 

 lake-surface. These distinct beaches would seem to indicate that the 

 rate of draining away and letting down of the water was not uniform, 

 but had periods of greater and periods of less rapidity. 



History of the Great Lakes. — The origin of these lake-basins is still 

 doubtful. They probably did not exist in the Tertiary period, but in 

 their place was a great depression draining northeastward. During 

 the Glacial epoch this depressed area was swept out and perhaps deep- 

 ened by the advancing ice-sheet. The irregular gouging of the ice- 

 sheet, and especially the irregular choking of the drainage area by 

 debris left by its retreat, probably gave origin to the lakes. In the 

 early Champlain, as already said, they were all united into one im- 

 mense sheet draining southward through the Ohio and Mississippi, 

 the natural drainage northeastward being prevented by the ice-foot. 

 Then, by the retreat of the ice northward, and the accompanying con- 

 tinental elevation, this one lake was broken up into several, which 

 found an outlet eastward through the Mohawk Valley and Hudson 



* Archives des Sciences, 1S89, vol. xxi, p. 355. 



