CLOUDS AND KIVEKS, ICE AND GLACIERS. 187 



have usually a good development of the structure. It 

 is finely displayed, for example, under the great medial 

 moraine of the glacier of the Aar. 



474. Upon this glacier, indeed, the blue veins were 

 observed independently three years after M. Guyot had 

 first described them. I say independently, because M. 

 Guyot's description, though written in 1838, remained 

 unprinted, and was unknown in 1841 to the observers on 

 the Aar. These were M. Agassiz and Professor Forbes. 

 To the question of structure Professor Forbes subse- 

 quently devoted much attention, and it was mainly 

 his observations and reasonings that gare it the im- 

 portant position now assigned to it in the phenomena 

 of glaciers. 



475. Thus without quitting the glaciers themselves, we 

 establish the connexion between pressure and structure. 

 Is there anything in our previous scientific experience 

 with which these facts may be connected ? The new 

 knowledge of nature must always strike its roots into 

 the old, and spring from it as an organic growth. 



§ 66. Slate Cleavage and Glacier Lamination. 



476. M. Guyot threw out an exceedingly sagacious 

 aint, when he compared the veined structure to the 

 cleavage of slate rocks. We must learn something of 

 this cleavage, for it really furnishes the key to the 

 problem which now occupies us. Let us go then to the 



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