of Mineral Species. 101 



minute crystals of sulphate, and is of variable thickness, in some 

 specimens more considerable than in others. Often, too, nothing 

 but the general outline of the original form is left, and large six- 

 sided pyramids or tabular prisms, as we are accustomed to find 

 them in witherite, shewing on their outside a drusy surface of nu- 

 merous crystals of heavy-spar, are found, when broken across, to 

 consist of the same species in aggregated crystals, generally in- 

 cluding cavities, from which the original species has disappeared, 

 and which have not been completely filled up. One of the spe- 

 cimens from Dufton, in Mr AI,T,AN'S cabinet, deserves a particu- 

 lar description. On a support of crystallized calcareous spar and 

 heavy-spar, the latter in rectangular tables of three inches in 

 length and upwards, are deposited the shapes of isosceles six- 

 sided pyramids, some of them two inches long, with a propor- 

 tional diameter, which were formerly witherite, but now pre- 

 sent a surface rough with crystals of heavy-spar, many of them 

 more than a line in length, and of course easily recognizable. 

 While the process of the transformation of carbonate into sul- 

 phate was going on, crystallized portions of the latter were like- 

 wise deposited on the surface, and particularly along the edges 

 of the original large tabular crystals of heavy-spar, where they 

 assume a position dependent upon the latter, and may be consi- 

 dered only as continuations of the same individuals. The se- 

 condary deposit, being of an opake milky whiteness, may be 

 readily distinguished from the transparent substance of the ori- 

 ginal crystals. These crystals themselves do not shew a homo- 

 geneous texture throughout. There are cavities inside of them, 

 often in such multitudes, that the remaining mass of heavy-spar 

 assumes a carious aspect, though still, by its cleavage, shewing 

 that it is part of the individual within whose external form it is 

 found. Many of the cavities are filled with small brown crystals 

 of calcareous spar. The crystallization of the calcareous spar, 

 begun in the form of the fundamental rhombohedron R, with 



