MODE IV. FELSITE. 16*5 



phyries, with a base of petrosilex of different 

 tints; and the little valley of Fierrozo, in which 

 flows the river Fersina, and at the entrance of 

 which is the village of Pergine, might be called 

 the valley of porphyries, from the infinite num- 

 ber of varieties which that rock there presents, 

 always preserving the same base. There are 

 grey, green, red, brown, black. There are some 

 similar to bricias, of a green cement with pieces 

 of other colours. The petrosilex in it gradually 

 passes to the granitic texture, and to th^ a 'tate of 

 schistose rock, without the beds changing their 

 direction."* 



STRUCTURE I. COMMON. 



Rose-coloured felsite, from Salberg, in Sweden. 

 This is interesting, as being the petrosilex of Wal- 

 lerius, which led to many errors of the Swedish 

 and French writers. He describes the following 

 kinds f. 



A scaly grey petrosilex, from Salberg. 



A scaly brown, from Garpenberg. 



The red scaly petrosilex, from Dalecarlia. 



The green scaly petrosilex, from the same. 



The blackish scaly, from Dannemora. 



He then describes several specimens of a waxy 



* Ib. 247, note, t i. 280. 



