Davis.] 
364 
[January 18, 
eroded rocks, which they follow with remarkable closeness ; 
and their recent conversion into lakes has been accomplished 
by local concentration of drift in deep, narrow valleys, where 
it could act effectively as a barrier ; as already stated, the 
present lake area does not mark the maximum attained during 
the decline of the ice-period. St. Mary’s River, Niagara and the 
St. Lawrence 1 are all post-glacial overflows after the obstruction 
by drift and the change of level by northern depression were 
accomplished. No more profitable field of work than the further 
study of this new topography can be, found for the amateur 
observer, and in the next ten years it should yield results of 
great interest. 
While much remains to be done to complete the history of our 
Great Lakes, it is certainly fair to say that sufficient possibilities 
of origin have been pointed out to show that there is no ne- 
cessity of considering them the result of' glacial excavation. 
Lake Erie may be considered fully explained as the effect of sim- 
ple subaerial erosion slightly modified by glacial action. The 
other lakes still present certain difficulties, but we are by no 
means therefore obliged to consider them the “ work of the great 
ice plow”; it is much bettei to admit our present ignorance of 
their precise origin, and hope for fuller explanation with further 
study and better knowledge of their surroundings. 
It has already been pointed out that rock-basins of moderate 
size, and presumably of glacial origin, are to be found among the 
numerous lakes of Northeastern America, but I believe they are 
far outnumbered by drift-barrier basins, and drift basins (C. 13). 
In Maine, most of the lakes are undoubtedly of this type 2 ; in New 
Hampshire they are also common 3 ; in the Adirondacks they 
seem to be very numerous 4 ; in Wisconsin, Lake Winnebago 
1 G. Maw (Geol. History of the N. American Lake Region, Geol. Mag. , v, 1878, 
455), recognizes certain new features in the St. Lawrence valley, and supposes without 
further evidence that the lakes above result from local post-glacial subsidence. 
2 C. E. Hamlin, On Mt. Ktaadn, Cambridge, Mus. Comp. Zook, Bulk, vu, 1881, 192. 
C. H. Hi chcock in the Scientific Survey of Maine, 1862, 297 and elsewhere, 
notes the abundance cf drift about many of the Maine lakes ; ledges are mentioned 
in the outlets of others. G. H. Stone, Glacial Erosion in Maine, Portland, Nat. Hist. 
Soc., Proc., 1881. 
8 C. H. Hitchcock, Geol. N. H., in, 1878,250. 
4 A good map but no description of cause is given by V. Colvin, Topogr. Surv. of 
the Adirondack Region, 1873-79. 
