242 FKAGMENTS OF SCIENCE. 



the point at which fracture begins to play a material 

 part. 



In ascending one of the larger valleys, we enter it 

 where it is wide and where the eminences are gentle on 

 either side. The flanking mountains become higher 

 and more abrupt as we ascend, and at length we reach 

 a place where the depth of the valley is a maximum. 

 Continuing our walk upwards, we find ourselves flanked 

 by gentler slopes, and finally emerge from the valley 

 and reach the summit of an open col, or depression in 

 the chain of mountains. This is the common character 

 of the large valleys. Crossing the col, we descend 

 along the opposite slope of the chain, and through the 

 same series of appearances in the reverse order. If the 

 valleys on both sides of the col were produced by 

 fissures, what prevents the fissure from prolonging 

 itself across the col? The case here cited is repre- 

 sentative ; and I am not acquainted with a single 

 instance in the Alps where the chain has been cracked 

 in the manner indicated. The cols are simply de- 

 pressions ; in many of which the unfissured rock can 

 be traced from side to side. 



The typical instance just sketched follows as a 

 natural consequence from the theory of erosion. Before 

 either ice or water can exert great power as an erosive 

 agent, it must collect in sufficient mass. On the higher 

 slopes and plateaus in the region of cols the power 

 is not fully developed ; but lower down tributaries 

 unite, erosion is carried on with increased vigour, and 

 the excavation gradually reaches a maximum. Lower 

 still the elevations diminish and the slopes become 

 more gentle ; the cutting power gradually relaxes, 

 until finally the eroding agent quits the mountains 

 altogether, and the grand effects which it produced in 

 the earlier portions of its course entirely disappear. 



