ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 183 



3. As a constituent of many elvans, as at Seveock and 



Terras. 



4. As distinct veins in kaolinized granite (Petuntzyte and 



Carclazyte) often associated with quartz, and sometimes 

 with tin. It is in such veins that the separate and 

 well-defined prisms already referred to usually occur. 



5. As globular radiate crystalline masses, in similar situa- 



tions. Some of these are so regular in form as to be 

 mistaken for pebbles.* 



6. As an important constituent of tin-capels, both in 



granite and killas — as at Wheal Uny, Dolcoath, and 

 other mines.f 



7. As fine green, brown, gray, or silver-white hair-like 



crystals (achroite) as a secondary deposit in schorl-rock;}: 



or imbedded in quartz crystals. § 

 The crystals of tin-stone lining minute fissures, joints, and 

 cavities ; which are of such importance in the numerous stock- 

 works in Cornwall, appear to be examples of local concentration 

 of previously existing material widely diffused in minute 

 particles throughout the rock-mass, produced by the operation of 

 the crystallizing forces. It has already been stated that the tin- 

 oxide present in the killas at Mulberry is little over one quarter 

 of one per cent. Were this distributed through the rock-mass 

 in particles as minute as those of the workable tin capels of the 

 Camborne district, it would be quite impossible to work it at a 

 profit. It is only because the crystals are comparatively large, 

 so allowing of much coarser stamping and much cheaper dressing, 

 that such poor deposits can be worked at all. 



* Dr. C. Le Neve Foster describes such masses as occurring, from one to five 

 inches diameter, in the decomposed granite of Ding Dong near Penzance. Trans. 

 R.G.S.C., IX, p. 9. See also Delabeche, Report, &c., p. 158. 



t See Cornish Tin-stones and Tin Capels. Plate iv and p. 11. 



J See Min. Mag., Vol. i, 1876, on the Achroite of Eock Hill, p. 55. 



§ The secondary development of schorl has been illustrated by Prof. Bonney 

 in the cases of LuxuUianite and Trowlesworthite, (Min. Mag. T, p. 15, and Trans. 

 E.G.S.C., X, 185), and its origin and general associations with Cassiterite, as well 

 as with Topaz and other fluorine bearing minerals, has been fully dealt with in 

 connexion with the hypothesis of Von Buch, Daubree, and others, by the writer 

 (Tin Stones and Tin Capels, p. 136 et seq.). 



