5 2 Origin of Granite. 



it is often a metamorphic rock, because it is some- 

 times impossible to draw any definite line between 

 gneiss and granite, for they pass into each other by in- 

 sensible gradations. About half-way up the Matter- 

 horn in the Alps, among the largely-contorted beds, a 

 thick stratum occurs, one end of which is true gneiss, 

 on the western side of the mountain, which striking 

 towards the eastern cliff, gradually gets more and more 

 crystalline till at length it passes into true granite. On 

 the largest scale, both in Canada and in the Alps, I have 

 frequently seen varieties of gneissic rocks regularly inter- 

 bedded with less altered strata, the gneiss being so crys- 

 talline, that in a hand specimen it is impossible to 

 distinguish it from some granitic rocks, and even on a 

 large scale the uneducated eye will constantly mistake 

 them for granites. Another very important circumstance 

 is that granite and its allies frequently occupy the 

 spaces that ought to be filled with gneiss or other rocks, 

 were it not that they have been entirely fused and 

 changed into granite. I therefore believe that many of 

 the granite rocks I have seen, are simply the result of the 

 extreme of metamorphism brought about by great heat 

 with presence of water. 



One reason why it has been inferred that granite is 

 not a common igneous rock is that, enveloping the 

 crystals of felspar and mica, there is generally a quan- 

 tity of free silica, not always crystallised in definite 

 forms like the two other materials. Silica being far 

 less easily fusible than felspar, it seems clear that had 

 all the substances that form granite been merely fused 

 like common lavas, the silica ought on partial cooling 

 to have crystallised first, whereas the felspar and mica 

 htave crystallised first, and the silica not used in the 

 formation of these minerals wraps them round often in 



