148 DOMAIN II. SILICEOUS. 



fon says*, there is in ^uvergne, near Sal vert, a 

 vein of quartz 10,000 fathoms in length. 



In the Uralian mountains it sometimes happens 

 that there is an entire mountain of quartz, another 

 of felspar, and a third of talc, thus presenting the 

 materials of granite on a very large scale. 



Aspect 2. Semi-transparent. This sometimes 

 forms very extensive veins, but perhaps never con- 

 stitutes an entire hill or rock. 



Aspect 3. Unctuous. This also appears in 

 veins, and generally accompanies metals. 



STRUCTURE II. GRANULAR. 



This kind often constitutes entire mountains in 

 Scotland, and other parts of the world. It has 

 sometimes been confounded with siliceous sand- 

 stone, but late writers have demonstrated that it 

 may be owing to a primitive but disturbed crystal- 

 lisation f. Nor is it inconsistent that where the 

 cement was deficient or interrupted, it should 

 appear among the most ancient substances in the 

 shape of mere sand. The grains, as already men- 

 tioned, are sometimes of the size of peas. 



* Min. i. 100. 



t Collate however the account of the Siliceous Sand-stones, 

 Mode XIV. Str. 2. 



