358 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



the alternate "zones" of tin and copper ores in the mines of 

 Cornwall, and concludes his remarks on this subject by saying 

 " it seems to me that no leaching out of metallic mineral from 

 the country walls will elucidate the problem."* 



If a deep-seated origin is thus indicated for sulphur and 

 for most of the metals occurring as sulphides, still more is it 

 called for in the case of tin which is so very local in its occurrence 

 yet so locally abundant ; so rare as a component of true stratified 

 rocks, so generally associated with eruptive rocks of the acidic 

 type, which have evidently been formed in the depths, and so 

 frequently an essential and in some cases apparently an original 

 component of such rocks. There are indeed cases where tin- 

 oxide exists disseminated in undoubtedly stratified rocks and in 

 large quantities, and not associated with any fissure-lode which 

 could be supposed to have been the channel of transmission 

 from the depth below, as for instance the tin stockwork at 

 Mulberry Hill already described. In these cases, however, it is 

 certain that the rock has been very far below the earth's surface 

 since its first formation and consolidation, and it seems probable 

 that the tin in such cases was an original constituent, deposited 

 from the waters with the rock material itself, and subsequently 

 concentrated for the most part, though not entirely, into the minor 

 fissures and shrinkage cracks as we see it. Had there been at 

 the proper time a fissure in this rock, i.e. while it was still 



* Ibid. It should perhaps be remarked here that though the succession of 

 zones of different minerals at difi'erent depth is notable enough, so that "the 

 richest copper lode (Dolcoath) in the county in 1840 is now the richest tin lode,'' 

 yet this alternation is often far too broadly stated, and in fact it cannot be said 

 to be established anywhere in the West of England except in the neighbourhood 

 of Carn Brea, and perhaps at the Phoenix Mines near Liskeard. In these 

 localities it is true to this extent. (1) The upper portions of the lodes were 

 worked for tin, little or no copper being present. But the gozzany chai-acter of 

 these portions plainly indicates the former existence of sulphides now removed, 

 and there is great reason to su^jpose that these sulphides were cupreous. (2) 

 Several of these lodes subsequently proved to be rich in copper and ceased to be 

 worked for tin. But there is reason to believe that notable quantities of tin were 

 still present, though lost in the abundance of sulphuretted minerals. An 

 examination of the burrows at Clifford Amalgamated shows that even there tin 

 occui-red with the copper. (3) It is certain that in the notable instances referred 

 to above the (probably) once mixed tin and copper ores have given place to tin 

 alone, the copper having altogether disappeared and the tin greatly increased in 

 quantity. 



