256 FRAGMENTS OF SCIENCE 



email to be visible ; while the essential facts and forms are 

 presented to the undistracted attention. 



A minute analysis of the phenomena strengthens the 

 conviction which the general aspect of the Alps fixes in 

 the mind. We find, for example, numerous valleys which 

 the most ardent plutonist would not think of ascribing to 

 any other agency than erosion. That such is their genesis 

 and history is as certain as that erosion produced the 

 Chines in the Isle of Wight. From these indubitable 

 cases of erosion — commencing, if necessary, with the small 

 ravines which run down the flanks of the ridges, with their 

 little working navigators at their bottoms — we can proceed, 

 by almost insensible gradations, to the largest valleys of 

 the Alps; and it would perplex the plutonist to ^:s. upon 

 the point at which fracture begins to play a material part. 



In ascending one of the larger valleys, we enter it wher« 

 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 upward, 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. Cross- 

 ing 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 pro- 

 longing 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 man- 

 ner indicated. The cols are simply depressions; in many 



