156 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



"Mineral Springs" have been observed in tbe Dolcoath lode, 

 but their characters have not been of any specially interesting 

 nature. The remarkable springs in the lode at Wheal 

 Seton a little to the north of the Dolcoath lode, and at Wheal 

 Clifford a few miles to the east, have been noted and described 

 by several writers, and particularly by Mr. J. A. Phillips. 

 These will be further referred to in a future chapter. 



"Local Chemical action," as occurring in the Perran lode, 

 has been already referred to. Similar action was formerly very 

 evident in many parts of Wheal Clifford in Grwennap. Further 

 reference will be made to this subject hereafter in connexion 

 with the theories of lode formation and of local metamorphism. 



Skc. 6. — The mutual relations of ore deposits and of country rooks. 



Many observations on this subject have been made by 

 previous writers on the phenomena of the West of England 

 ore-deposits In summarizing them in the present section, I 

 shall endeavour to add illustrations as may be necessary by 

 reference to specific examples. 



The following generalizations are almost universally 

 accepted. 



1. — Junctions of unlike rocks, and especially of eruptive 

 with stratified rocks (or proximity to such junctions) are regarded 

 as favourable conditions, the reverse as unfavourable.* Notable 

 illustrations of this statement are afforded by the Grreat Plat 

 Lode and the Caradon lodes, which are frequently bounded on 

 one side by graiiite and on the other by killas. A great many 

 examples of copper lodes locally enriched by proximity to elvan 

 courses are mentioned by Mr Henwood, and also by Messrs. 

 Barnett and Argall.f 



2. — Valuable deposits of tin and copper occur in both 

 eruptive and stratified rocks, but only in or near eruptive rocks 

 of the acidic type ; while valuable ores of lead, antimony, and 

 manganese only occur in stratified rocks, though usually in 

 proximity to basic eruptive rocks. 



*Mr. Chas. Thomas has recently stated this in axiomatic form for a certain 

 class of junctions : — " The junctions of granite and killas invariably {i.e. in the 

 West of England) exert a powerful infiaence for good on all metalliferous lodes." 

 — (Proc. Mining Assoc, and Inst. Corn.. 1883, p. 394. 



f On the Elvan Courses of Cornwall. — Report Miners Association, 1874, 



