110 Mr HAIDINGER on the Parasitic Formation 



the pseudomorphoses are supposed to have been formed, never 

 were seen or described by any mineralogist ; for instance those of 

 quartz in the shape of fluor from Beeralston ; those of hornstone 

 in the shape of calcareous spar from Schneeberg ; those of calce- 

 dony, in the shape probably of fluor, from Tresztyan in Transyl- 

 vania. We might be inclined to think, that actually there have 

 never been any, but that the new substance was formed while 

 the old one was disappearing. A film of quartz, deposited on 

 the surface of a crystal, would be the support of any new matter, 

 subsequently added, as we see in many instances, particularly 

 the pseudomorphous hornstone from Schneeberg, that, like the 

 inside, wherever it is not entirely filled up, the outside also of- 

 ten shews the reniform and botryoidal shapes depending upon 

 the undisturbed formation of the component individuals. Wa- 

 ter, charged with carbonic acid, and by that means holding si- 

 lica in solution, may have dissolved the original species, and de- 

 posited the siliceous matter in its stead. 



In the varieties from Schneeberg, which consist of perfectly 

 compact rhombohedral quartz or hornstone, the original outline 

 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 for- 

 merly occupied by calcareous spar, producing as many geodes 

 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 removed, leaving 

 the inner one in the shape of perfectly formed crystals of calca- 

 reous spar, the surface of which is stained brown by oxide of 

 iron. Mr ALLAN has one in his cabinet, which he disengaged in 



