283 



ON THE ORIGIN AND DEVELOPMENT OF ORE DEPOSITS 



IN THE WEST OF ENGLAND. 



By J. H. COLONS, F.G.S. 



Chap. V. — The relative ages of the West of England 

 Ore Deposits.* 



The facts set forth in the foregoing chapters, show clearly 

 that eruptive rocks (granite and " elvan " or felspar-porphyry) 

 have in some cases been forced into stratified rocks which were 

 already mineralized, as in the case of the tin stockworks in the 

 Killas. But these eruptives have themselves been broken 

 through by still newer eruptives (the later elvans), while mineral 

 veins of many kinds have been formed subsequently in the 

 complex so produced. These veins have been classed according 

 to their prevailing directions and varying contents as " elvans," 

 "lodes," "cross-courses," " flucans," "slides," "trawns," 

 " guides," &c. 



It is now generally recognised that the more important of 

 these veins nearly always occupy fault-fissures, and that they are 

 of very different ages. 



Nearly eighty years ago, Mr. Joseph Oarne read a most 

 important paper on this subject before the Eoyal Greological 

 Society of Cornwall, f In this communication, after drawing a 

 very just distinction between what were then called "contem- 

 poraneous veins (veins of segregation, &c.) and true fissure 

 veins (lodes, cross-courses, &c.), he gives examples of such veins. J 



* Continued from Vol. XII, p. 75. 



t On the relative ages of the Veins of Cornwall, Trans. Eoy. Geol. Soc. 

 Cornwall, 11, page 49 (Oct., 1818.) 



JThe following are examples of such veins : — 

 Veins of granite in granite. 



felspar in granite. 



mica in granite. 



schorl in granite and in-slate. 



schorl-rock in granite. 



quartz in granite and slate. 



