ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 141 



those given by Henwood. If we were to limit our measurements 

 to what are known as lodes in the old scientific sense, viz., the 

 actual fissiire filling, the mean width would probably be much 

 less for tin than for copper, since in general fissures are apt to 

 become narrower in depth for mechanical reasons, and most of 

 our deepest lodes now yield tin only, while nearer the surface 

 they yielded copper. On the other hand if we were to include 

 the workable portion of a vein whether it be lode-filling or 

 impregnated and mineralized wall, the mean width of tin veins 

 would come out much greater because, 1st, copper ores are rarely 

 so much disseminated as tin ores, and 2nd, if they were so, they 

 would not be workable lodes since they would not pay for 

 working. 



For other important general characters of lodes given by 

 Mr. Henwood, I must refer to his published works, those cited 

 above will su£B.ce for my present purpose. 



SPECIAL EXAMPLES OF "LODES." 



I now proceed to describe two of the more notable lodes in 

 the West of England, vi%., the great Dolcoath main lode yielding 

 ores of copper and tin, and situated in the immediate neighbour- 

 hood of the junction of granite and killas on the north side of 

 Carn Brea, and the still larger, though hitherto far less 

 valuable Perran main lode situated at a considerable distance 

 from the granite, and yielding chiefly iron and blende with a 

 little copper and lead. In these two lodes we shall find 

 illustrations of nearly all the phenomena of the lodes of the 

 West of England — the "gozzans," "leaders," and "capels;" 

 the "combed" and brecciated structures ; concretions; "vughs ;" 

 mineral springs and recent chemical deposits. 



(b) Dolcoath Main Lode. I select this lode partly for its great 

 importance and partly because its history is better known than 

 that of any other important lode in the West of England. It is 

 a great copper and tin lode, which has its outcrop in the slate 

 on the north side of Carn Brea Hill and passes downwards into 

 the granite, which rock is entered at depths varying from 80 to 

 120 fathoms from surface. It has been worked on at intervals 

 for a total length of 2|- miles, the greatest depth of the workings 

 is little short of 3000 feet, and it now yields nearly one-third 

 of the total tin raised in the West of England. 



