180 , PHYSIOGRAPHY. CLASS IT. 



between 68 38' and 68 53', the mean term of which is 68 

 45', as given above. Besides the forms mentioned there, he 

 likewise indicates an obtuse rhombohedron, in parallel posi- 

 tion with R, having a terminal edge of 1 79 30', which posses- 

 ses brilliant planes and sharp edges, but " is not easily re- 

 duced to the common laws of decrement." Obtuse rhom- 

 bohedrons of this kind, and also pyramids, occur in nature, 

 but they have not yet been sufficiently described. They 

 are easily explained upon the supposition of the series of 

 forms existing in the crystalline series of a species. If we 

 attend only to the principal series, we have R 8 = 178 

 47' in parallel position, R 9 = 179 23' in transverse 

 position, R 10 == 179 42', again in parallel position. 

 Considering the uncertainty of the angles of R, for Mr 

 PHILLIPS gives them as 69 12', and the same measure- 

 ment quoted in Mr BROOKE'S Introduction to Crystallo- 

 graphy is 69 30', and the difficulty of obtaining a perfect- 

 ly accurate result also from the admeasurement of the 

 obtuse angle, it is impossible to decide whether and which 

 member of the series the observed rhombohedron should be. 



2. The rhombohedral Euchlore-mica consists of 

 Oxide of Copper 39-00 . 58-00. 

 Arsenic Acid 43-00 21-00. 



Water 17-00 VAUCIUELIN. 21-00. CHENEVIX. 



It decrepitates before the blowpipe, is transformed into a 

 black spongy scoria, and then melts into a black globule, 

 having almost no vitreous appearance. 



3. It is found in copper veins traversing killas, associat- 

 ed with yarious ores of copper, particularly of the order 

 Malachite, also with ochrey varieties of prismatic Iron-ore 

 and rhombohedral Quartz. 



4. Several of the copper-mines in the vicinity of Red- 

 ruth in Cornwall, as Tingtang, Wheal Gorland, Wheal 

 Unity, &c. have yielded varieties of the present species. 



2. PRISMATIC EUCHLORE-MICA. 

 Kupferschaum. WERN. Letztes Mineral-System. S. 19. 50. 



