242 FKAGMENTS OF SCIENCE. 



navigators at their bottoms we can proceed, by almost 

 insensible gradations, to the largest valleys of the Alps; 

 and it would perplex the phitonist to fix upon 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 de- 

 pression in the chain of mountains. This is the com- 

 mon 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 pro- 

 duced by fissures, what prevents the fissure from pro- 

 longing itself across the col? The case here cited is 

 representative; 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. Be- 

 fore 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 tribu- 

 taries unite, erosion is carried on with increased vigour, 

 and the excavation gradually reaches a maximum. 

 Lower still the elevations diminish and the slopes be- 

 come more gentle; the cutting power gradually re- 

 laxes, until finally the eroding agent quits the moun- 



