depending on their Internal changes. ^ii 



filled up, the outside also often shows the reniform and botry- 

 oidal shapes depending upon the undisturbed formation of the 

 component individuals. Water, charged with carbonic acid, 

 and by that means holding silica in solution, may have dissol- 

 ved the original species, and deposited the siliceous matter in 

 its stead. 



In the varieties from Schneeberg, which consist of perfectly 

 compact rhombohedral quartz or hornstone, the original out- 

 line of the decomposed crystals of calcareous spar cannot any 

 longer be descried. There are varieties, however, also in the 

 shape of the same species, and consisting likewise of quartz, 

 where this is still possible ; and among them I know of none 

 that are more distinct than those from Bristol. The quartz, 

 in well defined individuals, is deposited partly inside the space 

 formerly occupied by calcareous spar, producing as many geo- 

 des or drusy cavities, and partly on the outside of the same 

 space, the two sets of deposits being separated by the surface 

 of the original crystal, the only thing still remaining of it. 

 They do not cohere firmly, but the outer deposit may be re- 

 moved, leaving the inner one in the shape of perfectly formed 

 crystals of calcareous spar, the surface of which is stained 

 brown by oxide of iron. Mr Allan has one in his cabinet, 

 which he disengaged in this way from the surrounding mass, 

 terminated on both ends, and altogether showing only a small 

 portion of its surface, where it might have been attached to an 

 original support. 



In the example just now described, the crystals of quartz 

 are deposited pretty regularly, at least in so far as their axes 

 are nearly perpendicular to the surface of the crystals of cal- 

 careous spar. This is not the case in the prismatoidal man- 

 ganese-ore from Ihlefeld, which fills up, and at the same time 

 surrounds, the space formerly containing crystals of calcareous 

 spar, and where likewise nothing but the surface of the origi- 

 nal crystals has remained. Both masses, however, are perfectly 

 alike, and consist of granular individuals, still easily recogniz- 

 able. Such component individuals are sufficiently small to 

 withdraw themselves from observation, in the varieties of com- 

 pact rhombohedral iron-ore from Johanngeorgenstadt in Sax- 

 ony, and other places, which exactly, like the manganese-ore, 



