ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 157 



3. — The character of the vein filling varies notably with 

 the nature of the country, as may be seen in the case of fissures 

 which pass through two or more kinds of rock. At Dolcoath 

 the lode was good for copper while in the soft killas of moderate 

 depths, but when it got into the harder killas near the granite 

 it was much poorer. At the 110 fathom level below the adit it 

 struck a trough of granite and began to yield tin, but was not 

 rich until Harriett's and the south lode fell in, the granite then 

 got softer and the lode wider and richer.* 



The Wheal Yor tin lodes are rich only while in the killas, 

 but die away or are entirely barren on entering the granite on 

 either side of the killas " trough." On the other hand the tin 

 lodes at Great Work, only a couple of miles away, are rich only 

 while in the granite and of very little value after entering the 

 killas. 



Similar examples in connexion with copper are afforded by 

 the Grwennap district. At Tresavean the lodes, and especially 

 the great lode, were worthless in the killas and of enormous 

 value in the granite. On the other hand the nearly parallel 

 lodes of the Consolidated and the United Mines in the same 

 parish were rich only in the killas and became worthless in the 

 granite. East Pool in the Camborne district was a notable 

 example of such change, for the main lode changed from rich 

 copper to rich tin in five fathoms, in passing from killas to 

 granite. 



Another very striking example is afforded by " Woolf's 

 cross-course in the Breage district. In the southern part of its 

 course near Pqrthleven, it yields lead at Wheal Rose — going 

 northward it is barren, containing only quartz and clay, in 

 passing through the Wheal Vor district which consists of a 

 different kind of killas — but still farther to the north in the 

 Grodolphin mines, where the rock changes again, it yielded very 

 large quantities of grey and yellow copper ores. Still another 

 example is afforded by the main tin lode of the Charlestown 

 mines near St, Austell, which in its western extension in the 

 granite and in the much altered " killas " yields only iron, while 

 farther east where it enters the fossiliferous Crinnis shales it 

 yields copper. 



*Josiah Thomas' Report Miners Association, 1882. 



