ORDER VI. PRISMATOIDAL KOUPHONE-SPAR. 



of the two species, because the fundamental forms in the 

 two cannot be united by any regular geometrical process, 

 and therefore belong to different systems of crystallisation. 



2. According to HISINGER, the prismatoidal Kouphone- 

 spar consists of 



Alumina 16-10. 



Silica 58-00. 



Lime 9-20. 



Water 16-40. 



Its chemical formula is Ca Si' 2 ~f 2 Al Si 3 + 12 Aq, which 

 corresponds to 8-77 lime, 15-82 alumina, 58-78 silica, and 

 16-63 water. Before the blowpipe it yields an opake vesi- 

 cular globule. It does not gelatinise with acids. 



3. The varieties of the present species and of the fol- 

 lowing one, agree very nearly in regard to their modes of 

 occurrence in nature, and are rarely met with, except 

 when accompanying each other. Their principal repositories 

 are the vesicular cavities of amygdaloidal rocks, and certain 

 metalliferous veins. In the first they are found deposited 

 upon the walls, which have often a coating of green-earth, 

 along with other species of the present genus, with 

 rhombohedral Lime-haloide, rhombohedral Quartz, &c. 

 Besides the metalliferous veins, where it occurs also with 

 various Kouphone-spars, and with ores of silver and lead, 

 it is found in metalliferous beds in primitive mountains, 

 with ores of copper and iron, and several species of the 

 genus Augite-spar. 



4. Magnificent crystals of a white colour are met with in 

 the vesicular cavities of the amygdaloids of Iceland and the 

 Faroe islands. Similar varieties have been brought also from 

 Indore in the Vendyah mountains in East India. Those from 

 the Tyrol are mostly compound, and of a brick-red colour. 

 Beautiful crystals of this colour occur near Campsie in 

 Stirlingshire, though the present species is less common in 

 Scotland and the Western Isles, than the following one. 

 The crystals from the silver-veins of Andreasberg in the 

 Hartz, are generally small, so are also those which occur in 



VOL. II. Q 



