162 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



is also unfavourable. The statement is however true apart from 

 this consideration, except when the hardness is due to silicification 

 and the production of schorl by the actual mineralizing circula- 

 tion, as is often the case with tin deposits. 



Since more or less of decomposition is a general phenomenon 

 in the rocks adjoining important ore-deposits, while silicification 

 or other impregnation-hardening apart from a definite deposit 

 of vein-quartz is only occasional, and in fact limited practically 

 in the West of England to tin deposits in killas,* it follows that 

 lead, copper, manganese, antimony, &c. are raridy found in 

 quantity in really hard strata, while even tin is often good where 

 the rocks have not been hardened locally. I proceed to give 

 some further specific illustrations of these statements. 



Tin in killas stockworks is mostly in a killas which is not 

 materially altered except by the production of white mica(G-ilbert- 

 ite), which accompanies the tin crystals occupying the joints. 

 Occasionally there is a little blackening due to the production of 

 schorl, and in a few instances crystals of quartz also occur. Of 

 course the stanniferous killas as a whole is a greatly changed 

 rock, but the final local change in the immediate neighbourhood 

 of the little veins is usually not at all strongly marked. It is 

 the stockwork as a whole which must be looked upon as the ore 

 body, and the contact metamorphism in connexion with it is 

 perhaps less than in any other instance known to the writer. 



At Old Wheal Vor, at Wheal Metal, and generally in the 

 Wheal Yor district, " when the rock is hard the lode produces 

 well, while soft strata are not congenial for that mineral [i.e. 

 tin ore) but often produce copper. "f 



The hardening in this neighbourhood is mostly silicification, 

 a great deal of very fine (microscopic) schorl exists in the rocks, 

 but not specially in immediate contact with the lodes. 



In granite, tin is usually accompanied by a local softening, 

 owing to the kaolinization of the felspar, although there is often 

 too a still more local hardening by the production of quartz or 

 schorl. 



* Siliceous infiltrations of the Silurian Strata in the Clog-au district of N. 

 Wales are always thought to be favourable for gold, while the soft beds are 

 unfavourable. See A. Dean, Report Miners Association, 1865. Of course the 

 association of gold with quartz is uniyersally recognized. 



fR. Hancock. Report M.A., 1870, p. 39. 



