166 GEOLOGY. 



In these granitoid tracts the most prevalent rock is 

 composed of quartz and large crystals of felspar, mica 

 being present, but only in very small quantities. The 

 hills have a most peculiar and characteristic appearance. 

 They are either huge crags with enormous cliffs, the sur- 

 face of which is not vertical, but curved, or they appear 

 as if composed of monstrous blocks, more or less rounded, 

 and piled upon each other. These peculiar appearances 

 are due to the mode of weathering ; the rock, when 

 unaltered, is so compact that water can only penetrate 

 through joints and cracks, and it is consequently in the 

 immediate neighbourhood of these alone that disintegra- 

 tion takes place. As a rule, all rain-water runs off the 

 surface of the rock, and consequently weathering is so 

 slow as to be almost imperceptible on the exposed portion. 

 The accumulation of a little soil and a few plants gene- 

 rally produces a hollow on the surface, and the rock in 

 contact with the soil is usually seen to have undergone 

 change. The result is that, while the exposed portions 

 of the rock remain unaltered, the surfaces of the joints 

 and cracks are gradually disintegrated and washed away, 

 untd the blocks, formerly separated by minute fissures, 

 become entirely isolated and detached. In this posi- 

 tion, so long as they arc exposed on all sides, they are 

 scarcely susceptible of change above, but disintegrate 

 slowly beneath, Avhere they are in contact with soil 

 and vegetation. 



and allied rocks, or to local conditions during the period of metamorphism. 

 As to granite of this kind oyer having been fluid, certainly such evidence as 

 I have seen during several years' study of tlie rocks is opposed to any such 

 supposition. 



