of Mineral Species. 109 



with a corroded surface, still contained within the covering, but 

 much diminished in size. A large pseudomorphosis in the shape 

 of a scalene six-sided pyramid, from the zinc mines in Somerset- 

 shire, in Mr ALLAN'S cabinet, from which the original species of 

 calcareous spar has entirely disappeared, is of a particularly inte- 

 resting nature. Beside the superficial coating, the quartzy matter 

 has introduced itself into the fissures of the crystal, parallel to its 

 planes of cleavage, and the interior of it is now not quite empty, 

 but divided into cells by lamella? of quartz, the cells having the 

 shape of the fundamental rhombohedron of calcareous spar. The 

 formation of what now remains 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 influence, by its electro-chemical action, upon the ar- 

 rangement and composition of the particles of that body, nl 



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 surrounded 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 oppor- 

 tunity of observing entire series of specimens illustrative of such 

 a process. Even in extensive collections, it is difficult to bring 

 together a sufficient number of them, in order to give an ex- 

 ample of each stage of the gradual formation and decomposition 

 of one species after the other. The moulds in which many of 



