DOMAIN I. SIDEROUS. 



felspar is very common upon the banks of 

 Isere, and this is not extraordinary, as there are 

 even entire mountains of it in Dauphiny, which I 

 have myself seen. The famous silver mine of 

 Challenches, into which I descended, is in a moun- 

 tain of this kind. The varieties of this rock are 

 extremely numerous ; we find it with leaves sin- 

 gularly distorted, or bent in zigzag. It sometimes 

 occurs with thick leaves, and at others as thin as 

 paper. In some varieties, the leaves of pure and 

 coloured hornblende, alternate with leaves of white 

 and pure felspar ; in others, these two substances 

 are almost confounded; in others, in short, the 

 leaves are interrupted either suddenly or by gra- 

 dations. There are also frequently found knots 

 or detached pieces of white felspar, confusedly 

 crystallised, and often mixed with quartzose parts. 

 It is 'curious to observe, when these knots are of 

 irregular shapes, the exactness with which the 

 schistose leaves follow all the convolutions of the 

 knots, and form a kind of fortifications around 

 them. 



" The hornblende varies in its colour; some- 

 times black and brilliant, sometimes inclining to 

 green, at others brown or grey ; its form some- 

 times presents pretty regular crystals, especially 

 in schisti, whose leaves are straight ; and at other 

 times thin plates, almost as brilliant as mica. 



