314 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
be at its maximum, Wallace examines several lakes to discover if 
those of glacial origin have not some distinctive feature by which 
they can be recognized. He points out that greater length than 
breadth, and simplicity of outline, are highly significant of glacial 
erosion, and that the absence of lateral bays and branches is strongly 
against the theory of warping or submergence. In this connection 
it is recognized that “ the lake surface , not the lake bottom , repre¬ 
sents approximately the level of the preglacial valley, and that the 
lateral streams and torrents enter the lake in the way they do 
could only erode their channels down to the level of 
the old valley before the ice overwhelmed it.In connection 
with this subject may be noticed the many cases in which Alpine 
valleys present indications of having been greatly deepened by 
glacial erosion, although, owing either to the slope of the ground or 
to the uniformity of the ice-action, no lake has been produced. In 
some valleys, as in that of Lauterbrunnen, the trough between the 
vertical rock-walls was probably partly formed before the ice age, 
but was greatly deepened by glacial erosion, the result being that 
the tributary streams have not since had time to excavate ravines 
of equal depth with the main valley, and therefore form a series of 
cascades over the lateral precipices, of which the Staubbach is the 
finest example. In many other cases, however, the side streams 
have cut wonderfully narrow gorges by which they enter the main 
valley” (’ 93 , 754, 768). 
McGee's Second Paper on Glacial Canyons , 189f — McGee has 
given a fuller statement of the action of glacial erosion in produc¬ 
ing discordance between lateral and main valleys in a second essay, 
again entitled “ Glacial Canons,” published eleven years after his 
first essay on this subject. After a discussion of glacial erosion 
in general, it is stated that “ Glacial canons are characterized by 
several peculiar features: 1. They are U-shaped rather than V- 
shaped in cross-profile; 2. Small tributary gorges usually enter at 
levels considerably above the cahon-bottoms; 3. In longitudinal 
profile the canon-bottoms are irregularly terraced,— i.e.,made up of 
a series of rude steps of variable form and dimensions,— and some of 
the terraces are so deeply excavated as to form rock-basins occupied 
by lakelets.In a region of rapid corrasion then, the main 
[water] stream must.... more rapidly corrade its channel than does 
its minor tributary; and the tributary canon must accordingly enter 
its principal over a rapid or at least a convex curve in longitudinal 
profile. If now the main canon become filled with ice and be 
because they 
