depending on their Internal changes. 291 



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, showing on their out- 

 side a drusy surface of numerous crystals of heavy-spar, are 

 found, when broken across, to consist of the same species in 

 aggregated crystals, generally including cavities, from which 

 the original species has disappeared, and which have not been 

 completely filled up. One of the specimens from Dufton, in 

 Mr Allan's cabinet, deserves a particular description. On a sup- 

 port 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 proportional diameter, which 

 were formerly witherite, but now present 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 sulphate was going on, crys- 

 tallized portions of the latter were likewise 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 considered only as con- 

 tinuations of the same individuals. The secondary deposit, 

 being of an opaque milky whiteness, may be readily distin- 

 guished from the transparent substance of the original crys- 

 tals. These crystals themselves do not show a homogeneous 

 texture throughout. There are cavities inside of them, often 

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

 sumes a carious aspect, though still, by its cleavage, showing 

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

 found. Many of the cavities are filled with small brown crys- 

 tals of calcareous spar. The crystallization of the calcareous 

 spar, begun in the form of the fundamental rhombohedron R, 

 with yellowish-white faintly translucent matter, as appears 

 from the delineation of colours, was completed by a brownish 

 opaque matter, in the shape of the combination R — 1 . R + oo, 

 the form dodecaedre of Haiiy. These brown portions have 

 also a carious aspect, as from decomposition, and are studded 



