32 GRANITIC ROCKS OF OTHER COUNTRIES [Ch. III. 



Zinnwald the entire hill is composed of granite and quartz- 

 rock, disposed in regular alternating beds. The granite is 

 fine-grained, and consists of felspar, quartz, and mica, all of 

 which are white : it is, however, rarely found in a perfect, 

 unaltered state, and that only near the centre of its beds: 

 more commonly the mica passes into talc, and its felspar is 

 decomposed into kaolin. These granitic beds vary from three 

 to ten fathoms in thickness, and do not contain a trace of tin 

 ore. The quartz-rock is of two kinds : the one is nearly pure 

 quartz, grey, crystalline, with a greasy aspect, containing dis- 

 seminated scales of mica, and crystals of wolfram and oxide 

 of tin ; the other, the greisen of the Germans, and hyalomicte 

 of the French, is a granitic mixture of grey, crystalline quartz 

 and argentine mica in large scales. Its beds are as thick as 

 those of the granite : they contain disseminated masses of 

 quartz and of granite, and abound in oxide of tin. The alter- 

 nating beds of these rocks are inclined at angles varying from 

 15 to 20. The beds of compact quartz-rock are commonly 

 enveloped in those of the granitic variety, and thus alternate 

 with the beds of granite: on the one hand, the granitic 

 quartz-rock passes into the compact, by an augmentation in 

 the size of the quartz grains, and by the coalescence of the 

 mica into larger scales, and its diminution in quantity ; and, 

 on the other hand, it becomes finer grained, and graduates 

 into the granite.* 



This short sketch shows that the granite in the Erzgebirge, 

 as in Cornwall, is disposed in detached and insulated masses, 

 and that these likewise do not possess the same mineral com- 

 position. In the next place we learn, that the different rocks 

 of which these masses are composed also affect a similar 

 arrangement in beds which sometimes alternate ; and it is 

 worthy of remark, that in the granite of Zinnwald, which 

 decomposes into china-clay, talc, not mica, is the characteristic 

 mineral, which is precisely the condition of the granite in 

 Cornwall wherever this substance abounds. The nature of 



* Annales des Mines, tomes 8. et 9. 



