120 OEIGIN AND DEYELOPMENT OF ORB-DEPOSITS. 



Stockworks in the West of England have I believe only 

 been worked for tin and for copper. Tin stockworks have been 

 worked in ordinary " killas," and in that modification of it known 

 as tourmaline schist ; also in granite and several of its modifica- 

 tions, as greisen and schorlyte ; and in several kinds of felspar- 

 porphyry (elvan). Copper stockworks have been worked in 

 killas and in granite. 



Some of the larger tin stockworks in killas, as for example 

 Mulberry and Minear Downs, seem to be very nearly related to 

 the contemporary ore-beds already described, since they are often 

 entirely unconnected with anything like a definite workable lode, 

 and only very rarely with one that will pay of itself for working. 

 But the strike of the belt of the impregnated ground is not 

 necessarily or usually that of the general country rock, while its 

 dip is usually much steeper than that of the beds. The chief 

 individual strings are usually still more steeply inclined or even 

 vertical, and, as will be shewn hereafter, these strings contain a 

 notable proportion of the whole mineral contents of the belt, 

 and nearly the whole of that which is extracted or extractable by 

 the simple methods in use, and which are nevertheless in most 

 cases the only ones economically possible. 



The tin stockworks in altered granite usually conform in 

 strike to the direction of the nearest junction with killas, but 

 these also seem to be often unconnected with definite lodes. 



Examples illustrating each of these varieties of "stockwork" 

 will be described in the following order : — 



1. Tin stockworks in killas apparently unconnected with 

 any workable lode. Examples, Mulberry, Wheal Pros- 

 per, Minear Downs. 



2. Tin stockworks in killas plainly connected with workable 

 lodes. Examples, Great Wheal Fortune, Pednandrea. 



3. Tin stockworks in "granite" unconnected with lodes. 



Examples, Carrigan, Cligga. 



4. Tin stockworks in granite connected with lodes. Exam- 

 ples, Beam, Balleswidden. 



5. Tin stockworks in "elvan." ^:i;«w^/(?s, WhealJennings, 



Terras. 



