d^ Mr Haidinger on the Parasitic Formation of Miner ah ^ 



is now not quite empty, but divided into cells by lamelLx^ of 

 quartz, the cells having the shape of the fundamental rhom- 

 bohedron of calcareous spar. The formation of what now re- 

 mains must have begun, therefore, when the original crystal 

 was still perfect, and have proceeded during the decomposition 

 of it. The change was gradual, and so we must conceive these 

 processes to go on in every instance. It is highly probable 

 that the formation of another species, so near, or even within 

 the boundaries of a crystal previously existing, will greatly in- 

 fluence, by its electro- chemical action, upon the arrangement 

 and composition of the particles of that body. 



Quartz, more than any other species, is known to fill up the 

 vacuities formerly occupied by crystals of calcareous spar, of 

 fluor, and of gypsum. Such masses of secondary formation 

 are called pseudomorphoses, and are usually conceived to have 

 been formed in moulds, arising from a substance which sur- 

 rounded the original crystals, and was left unchanged, while 

 the latter was destroyed by decomposition, in a manner similar 

 to the process of making first the mould of a bust or statue, 

 and then filling it with plaster of Paris. The cast obtained, 

 from a mineralogical point of view, is a pseudomorphosis of 

 gypsum. We have but rarely an opportunity of observing 

 entire series of specimens illustrative of such a process. Even 

 in extensive collections, it is difficult to bring together a suffi- 

 cient number of them, in order to give an example of each 

 stage of the gradual formation and decomposition of one spe- 

 cies after the other. The moulds in which many of the pseu- 

 domorphoses 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 horn- 

 stone, in the shape of calcareous spar, from Schneeberg ; those 

 of calcedony, in the shape probably of fluor, from Trestyan in 

 Transylvania. 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 hornstonc from 

 Schneeberg, that, like the inside, wherever it is not entirely 



