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POPULAR SCIENCE REVIEW. 
the north-west of England, and of North America. Some of 
these are moderately deep, others rather shallow, some of 
gigantic dimensions, others only occupying a few square miles. 
The causes assigned for their origin are: (1) Denudation acting 
slowly in the ordinary way; (2) Grlaciers pushed forward by 
a great body of ice from behind, and ploughing out large 
shallow hollows in soft rocks ; (3) Natural valleys choked at 
some point either by detritus brought by water or ice, or by 
geological disturbances ; (4) Natural hollows caused by faults, 
dislocations, and other results of elevation of the mass. It 
will not be denied that in a certain sense and to a certain extent 
each of these may be regarded as a vera causa. The question 
is, how far in a particular case any one of these agencies, as, 
for instance, denudation or ice, has been engaged in completing 
or doing the essential part of the work. Professor Ramsay, 
following in the steps of some ingenious Swiss geologists, has 
gone so far as to teach that lake basins generally are due to 
the erosive power of ice, and to that power only. He is sup- 
ported in some measure by Sh* W. Logan, who believes that 
the great American lake basins are results of denudation, not 
of geological disturbance. That they and the rest of the great 
plains, as well as the mountains and hills of the Alps and of 
Northern America, besides those of Northern Europe, have been 
affected by denudation and by ice, may be said to be certain. 
That there has been enormous elevation to' produce the moun- 
tains and remove the vast mass of material once accumulated 
on rocks now forming the topmost peaks, is as certain as that 
the peaks are there ; and that there have been in the valleys 
huge glaciers, compared with which existing glaciers of the 
Alps are as nothing, is not less true. But neither is it less 
true that there has been great elevation; that great elevation, no 
matter how long it has taken to complete it, must have resulted 
from and taken place in obedience to mechanical laws, and that 
the production of fissures, of faults, of axes, and of occasional 
wide interspaces or valleys between rocks, is inevitable. 
Now, if we look at any of those lakes that have been formed 
in mountain districts, we shall find that they occupy valleys or 
portions of valleys, having a distinct relation to the great 
mountain systems adjacent. The Alps on both sides abound 
in such valleys, and very marvellous they are, exhibitiug marks 
that cannot be mistaken of disruption and of erosion ; disrup- 
tion originally producing crevices, and erosion tearing away, 
enlarging, and widening these natural clefts to such an extent 
that their original character is lost and obliterated. 
The reader is requested to cast his eye over the various 
mountain lakes whose forms are delineated on the accompany- 
ing plate. The Lake of Como is long, straggling, and forked; 
the sister lakes have the same general features. They have 
