224 FKAGMENTS OF SCIENCE. 



The annexed sketch of a model (p. 225) will enable 

 the reader to grasp the essential features of the problem 

 and its solution. Glen Gluoy and Glen Roy are lateral 

 valleys which open into Glen Spean. Let us suppose 

 Glen Spean filled from v to w with ice of a uniform 

 elevation of 1,500 feet above the sea, the ice not 

 filling the upper part of that glen. The ice would 

 thrust itself for some distance up the lateral valleys, 

 closing all their mouths. The streams from the moun- 

 tains right and left of Glen Gluoy would pour their 

 waters into that glen, forming a lake, the level of which 

 would be determined by the height of the col at A, 1170 

 feet above the sea. Over this col the water would flow 

 into Glen Roy. But in Glen Roy it could not rise higher 

 than 1150 feet, the height of the col at B, over which it 

 would flow into Glen Spey. 



The water halting at these levels for a sufficient 

 time, would form the single road in Glen Gluoy and the 

 highest road in Glen Roy. This state of things would 

 continue as long as the ice dam was sufficiently high to 

 dominate the cols at A and B ; but when through change 

 of climate the gradually sinking dam reached, in succes- 

 sion, the levels of these cols, the water would then begin 

 to flow over the dam instead of over the cols. Let us 

 suppose the wasting of the ice to continue until a con- 

 nection was established between Glen Roy and Glen 

 Glaster, a common lake would then fill both these glens, 

 the level of which would be determined by that of the 

 col C, over which the water would pour for an indefinite 

 period into Glen Spean. During this period the second 

 Glen Roy road and the highest road of Glen Glaster 

 would be formed. The ice subsiding still further, a 

 connection would eventually be established between 

 Glen Roy. Glen Glaster, and the upper part of Glen 

 Spean. A common lake would fill all three glens, the 



