OKiaiN AKi) DEVELOPMENT OF ORE-BEPOSITS. 347 



(h.) Chalcedonic, in concretionary or stalactitic forms, as 

 red or brown jasper, as an opaque milk-white mass, as chert, 

 &c., all anhydrous, and with specific gravity over 2-6. 



[p.) Opaline, as pure opal, semi-opal, mangan-opal, ferrugi- 

 nous opal, &c., all hydrous, and with specific gravities varying 

 from 2*2 upwards. 



As to the origin of the different modifications of the first 

 group, it would appear that they have been formed almost 

 everywhere in the West of England and at all periods, from the 

 very earliest down to or near to the present time. Except 

 perhaps in the case of the cross-course spar, it is likely that 

 these forms of quartz have been produced very slowly. The 

 second group is much more local, but it occurs in many situations 

 under conditions which suggest a rather rapid deposition. Prob- 

 ably its origin is directly connected with thermal springs. 



The third group or opaline variety was also no doubt 

 deposited by thermal springs and probably at a very high tem- 

 perature. It is scarcely too much to say that in the whole of the 

 West of England mining • region, a rock which has not been 

 infiltrated with some form of silica, since its first consolidation, or 

 a fissure in which silica has not been deposited at more than one 

 epoch, is a very rare exception. This being the case, we can 

 hardly expect any direct connexion of origin to be traceable 

 between the silica and the associated ore. There is quartz with 

 the tin veins in the granite, but there is precisely similar quartz 

 without tin ; siliceous tin and copper capels abound, but there are 

 very similar capels without tin or copper. Iron is found with the 

 silica in some cross-courses, but in others it is absolutely free 

 from iron. 



Even the frequently observed connexion of silica with gold''* 

 is little noticeable in Cornwall, although that metal has been 

 found in cross-courses in Breage, near Eedruth, and at Poltimore 

 in Devon. 



* Interesting examples of recently-formed siliceous deposits containing gold, 

 silver, cinnabar, and other metallic minerals, are described by Dr. Robert 

 Oxlandand Mr. J. A. Phillips {Phil. Mag., Nov. 1868). 



Mr. Dean remarks, " The silurian beds traversed by the gold lodes (in the 

 Clogau district, where some very profitable mines were formerly worked) when 

 hard and sharp and well silicified are the best for gold, the soft beds are imfavour- 

 able (" Notes on Gold Mining in Wales," JSep. M.A., 1865.) 



