312 GEOLOGY. 



By the disintegration and decomposition, the fel- 

 spar of granite is changed into clay, the quartz 

 grains into sand, while the mica is broken down and 

 mixed with the clay and sand. 



Basalt and Lava. 



Dolomieu observes, with respect to basalt, that 

 many subjects of different natures so much resemble 

 each other, in analysis as well as in appearance, that 

 it is difficult to fix the application of the name to any 

 one in particular. That all basalt has been in a state 

 of fusion is highly probable, it melts at such a low 

 temperature. 



Basalt affects the magnetic needle, containing about 

 twenty per cent, of iron, besides many heterogeneous 

 substances, but it is not otherwise metalliferous. 

 When exposed to the weather, it gradually crumbles 

 down into a fine black mould, which constitutes a 

 very fertile soil, and it is to this rock that some of 

 the richest parts of Scotland owe their fertility. 



Lava retains its heat for a very long period of 

 time. Spallanzani saw a piece of wood take fire in 

 lava three years and a half after it was thrown out, 

 and at a distance of two leagues from the crater.* 



Whinstone and Lava. 



Sir James Hall completely established the identity 

 of whinstone and lava. He also ascertained that 

 carbonate of lime might be readily fused if it were 

 under a pressure equivalent to about a mile and a 

 half of sea. 



* The horizontal stratification of basalt, and its gradation 

 downwards through the softer substance, wacke into clay, con- 

 vinced Werner that it was not a volcanic product. Von Buch, 

 on the other hand, considers basalt and porphyry as the dwelling- 

 place of volcanoes. 



