1882 .] 
857 
[Davis. 
not allow. Undoubtedly, however, the glacier did have some 
effect, and in so far as it aided the formation of the lake, the 
basin would be classed with those of glacial erosion. 
3. At an early period of the mountains’ history, they may 
have stood higher, so that M instead of G should be the level of 
'drainage discharge; at such a time the valley AE D F would be 
the result of simple river erosion. A later subsidence would 
bring the sea-level to E C G K, and the depression would grad- 
ually fill to that level with detritus from the higher mountains. 
After this an advance of glaciers from the upper valleys is sup- 
posed by de Mortillet to sweep out the sediments from the 
lake-basin, EDO, and in evidence of this he points to the 
superposition of terminal moraines at C on stratified deposits such 
as cover the basin G. This imolies a much less powerful erosion 
than the preceding, and is to that extent more admissible, but I 
doubt whether even this amount is not too great. 1 Most assuredly 
all Swiss lakes of preglacial origin have had a new lease of life 
given them by this glacial cleaning-out, whether it was partial or 
perfect. 
The three explanations all agree in considering the deposits 
H G of preglacial date, or pleiocene. The question has been raised 
whether the two may not be in part contemporaneous, 2 and this 
suggests a fourth sivpposition. 
4. As in number 3, the erosion of A E D F was accomplished 
during a period of greater elevation than the present and deposits 
H M were then accumulated; during the latter part of the same 
period the old extension to F of the Alpine glaciers occurred. 
A subsidence then began, and keeping pace with it, the deposits 
M G were laid down ; but EDO being occupied by ice, was pre- 
served from filling with sediment, and on the melting of the ice 
was left empty (C. 13). The occurrence of glacial moraines 
1 It should be noted that however applicable de Mortillet’s theory may be to these 
lakes and others in Switzerland, it leaves unexplained the origin of the rock-basin 
itself, such as that of Lake Geneva; and also that, according to Riitimeyer (Pliocan 
und Eisperiode, 8) the gravels under the moraine by Como are not derived from the 
upper valleys of the Alps, but from the neighboring hills, and therefore give no proof 
that Lake Como was ever filled with detritus. 
2 Desor, Paysage Morainique, Paris, 1875, 30. Later observations make this doubt- 
ful. 
