ORDER VII. UNCLEAVABLE QUARTZ. 335 



varieties generally exhibited among the latter do not, how- 

 ever, belong to the present species, but to the preceding 

 one, being Hornstone. Opal itself is subdivided into Pre- 

 cious Opal, containing the varieties which shew the play 

 of light ; into Wood Opal^ which appears in the shape of 

 trunks, branches, and roots of trees ; into Common Opal and 

 Semi-Opal, consisting of the rest of the varieties, and distin- 

 guished from one another by the higher or lower degrees 

 of transparency and lustre, and of the perfection of their con- 

 choidal fracture. Opal-Jasper evidently also belongs to the 

 species of uncleavable Quartz, for it contains varieties that 

 are in the same relation to it, as Jasper is to the species of 

 rhombohedral Quartz. Hydrophone is nothing but a va- 

 riety of Opal without transparency, but assuming it if 

 thrown into water or another transparent fluid. Siliceous 

 sinter is a deposit from hot springs, &c. and according to 

 its specific gravity, seems to belong to the present species. 



3. Three varieties of uncleavable Quartz, 1. Hyalite, 

 analysed by BUCHOLZ, 2. Precious Opal, and 3. ?lenilite, 

 both analysed by KLAPROTH, have yielded 



Silica 1. 92-00 2. 90 00 3. 85-50. 



Water 6-33 10-00 11-00. 



The last contains, like several other varieties, a small pro- 

 portion of oxide of iron, alumina, lime, and carbon. Opal 

 Jasper contains so much as 47 per cent, of oxide of iron. 

 The contents of water are considered by BERZELIUS fo- 

 reign to the mixture of the mineral, and to change with 

 the hygrometric state of the atmosphere. Before the blow- 

 pipe, water is disengaged, the mineral decrepitates and be- 

 comes opake, and shews besides the properties of pure sili- 

 ca. Two pieces rubbed together give a phosphorescent 

 light, like rhombohedral Quartz. 



4. Uncleavable Quartz is less frequently met with in 

 nature than the preceding species. Generally it forms 

 short irregular veins, strongly connected with the matrix, 

 which in most cases is porphyry, or is imbedded in it in 

 amorphous masses of various dimensions. If they be lar*ge, 

 they sometimes assume the shape of more or less regular 



