186. 



OF COMPOUND MINERALS. 



from rhomboheclral Lime-haloide, from octahedral Fluor- 

 haloide, from prismatoidal Gypsum-haloide, &c. ; which is 

 sufficient to prove, that the forms of the pseudomorphoses 

 cannot by any means be members in the series of crystal- 

 lisation of those species (. 136.), to which they belong. 



The quality of the surface of the pseudomorphosis depends 

 only upon its form, and not upon its substance or its mode 

 of composition. For the elevations and depressions of the 

 mould are likewise expressed in the cast, which in this case 

 is the pseudomorphosis. The quality of this surface tends 

 very often to indicate the mineral, from which the form is 

 derived, particularly if these forms belong to the tessular 

 system, which occurs in several species. Thus rhombo- 

 hedral Quartz presents not unfrequently pseudomorphic 

 hexahedrons, which as such may originate from various 

 minerals. But on a closer inspection, we observe upon the 

 faces of some of them, the obtuse apices of isosceles tetra- 

 gonal pyramids, which, as it is mentioned in . 176., belong 

 to the hexahedrai tetragonal-icositetrahedron, a form not 

 uncommon in octahedral Fluor-haloide. "We may hence 

 infer that the form of the pseudomorphosis of rhombohe- 

 dral Quartz now under consideration, is owing to the spe- 

 cies of octahedral Fluor-haloide. 



The surface of the pseudomorphosis is never drusy in 

 the sense of . 176., but only in the way described above. 

 But sometimes it bears a new coating of very minute crys- 

 tals, of the species of which the pseudomorphosis consists. 

 This is not at all uncommon in many of those pseudomor- 

 phoses of rhombohedral Quartz, which affect the form of 

 rhombohedral Lime-haloide. In general the surface of the 

 pseudomorphoses is less smooth and shining than that of 

 real crystals of the species. This, however, is merely ac- 

 cidental, and does not deserve to be classed among the pe- 

 culiar and constant characters of pseudomorphoses. 



The pseudomorphoses are very often hollow in their in- 

 terior ; the cavities are lined with crystals, or with reni- 

 form and other imitative shapes of that species, which con- 

 stitutes the pseudomorphoses. There are crystals which 



