ORIGIN AND DEVELOPMENT OF OKE-DEPOSITS. 367 



With all that is here said I entirely agree, but it seems to 

 me that several of the phenomena of such stockwork deposits 

 become clearer and easier to understand, if we suppose that the 

 rocks in which the fissure has been opened was already charged 

 with tin in some form, disseminated through the mass, and 

 perhaps even concentrated into shrinkage cracks before the 

 opening of the fissure. 



If there is reason to believe that the killas stock works 

 represent rock masses permeated with tin, and that at Wheal 

 Music with copper, before fissuring, much more is this the case 

 with such granite stockworks as those at Oarrigan and Eock 

 Hill, and with such stanniferous elvans as that at Wheal 

 Jennings. The same may be said of the copper stockworks at 

 Wheal Yyvyan. But these of course are known to be of much 

 more recent origin. 



Capels. The substances known in Cornwall and Devon as 

 capels may be described as highly altered and usually silicified 

 bands of country rock, bordering a more or less distinct fissure 

 or fissure-filling. The term is sometimes applied to a silicified 

 or mineralized band at the side of a fissure traversing granite 

 or even elvan, but most well-marked capels occur in killas.* 

 Fig. 16, plate ix, illustrates one form of capel in killas. 



Tin capels contain particles of tin-oxide in notable propor- 

 tions, though not always in quantities suffioient to pay for 

 working. The contents of the actual fissures (the leaders) in 

 these cases are often quite unimportant from the miner's point of 

 view, as in the case of the greater portions of the tin lodes on 

 each side of Carn Brea Hill. In all the mines of this district 

 the chief part of the tin is obtained from the capel, which 

 extends sometimes for several fathoms on each side of the 

 fissure or lode proper, which is sometimes a mere joint in the 

 rock. 



* The word capel (at Penhalls) is applied to a rock which appears to me to be 

 simply an altered killas, a killas which has been greatly acted on by mineral 

 solutions and changed from a soft slaty rock into a hard dark-coloured compact 

 mass of quartz and schorl : these minerals being arranged in streaks following 

 the original lines of stratification of the killas. In addition the capel is generally 

 full of short lenticular veins of quartz, and is intersected by numerous little 

 strings of cassiterite and chlorite. Foster, Trans. Roy. Geol. Soc. Corn., ix, 

 207. 



