304 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



These all strike South of West, although of the sub-group h, 

 some turn very much northward near Perran and Penryn. In 

 the main all the sub-groups strike with the strike of the rocks, 

 although the dips or underlies are often diiferent.* 



2. St. Austell series. General direction, a few degrees 

 N. of W. Sub-group a, St. Mewan, N.N.E. These elvans 

 mostly make acute angles with the strike of the rocks. 



3. E.W. series, St. Neot to Watergate Bay. These mostly 

 run nearly or quite in the strike of the rocks. 



4. E.W. series, Bodmin granite to Dartmoor. These also 

 correspond pretty accurately with the strike of rocks. 



Although the elvans cut indiscriminately through stratified 

 and unstratified rocks, they can rarely be traced very far into 

 the hearts of the various granitic masses, f and when they are 

 so formed, their mineral character is usually much altered, more 

 particularly they are often seen to be fine-grained and much 

 decomposed. 



The veins of fine-grained granite which frequently cross 

 the ordinary granite, and the " veins of granite " in the lode at 

 Wheal Vyvyan, are probably faults with granite fillings just like 

 the ordinary elvans. These still remain as lines of weakness in 

 some instances, as for example, at W est Gwallon and Ting Tang, 

 where the lodes for some distance have a wall of elvan on one 

 side and of killas on the other. Probably many other such 

 instances exist, but are undiscovered because of the absence 

 of mineral riches in the secondary fissure, as already stated. 

 Many of the elvans are notably stanniferous or cupriferous. 



Class III. The Oldest Tin Lodes. 



Of lodes older than the bulk of the elvans, very few exam- 

 ples are certainly known, but they are probably more numerous 

 than is generally supposed. Those that are known, invariably 



*" It is not usual for the strike and di'p of elvan-courses to run parallel with 

 those of the laminae of the schistose rocks, this coincidence, however, occurs at 

 Herland." Henwood, V, 115. 



t It has been suggested as an explanation of the difficulty of tracing the 

 elvans into the granite masses, that perhaps these masses were still soft and pasty 

 in the interior, and so could not remain open as fissures at the time when the 

 elvans were injected. 



