ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 145 



between it and Cam Menelez. The lode approximates more to a 

 plane than the actual junction ; consequently, for short distances 

 it has sometimes both walls of slate, and at others both of 

 granite. But it is evident that the position of the great lode has 

 been in the main determined by the line of weakness due to this 

 junction.* 



It would appear that the " wave-like succession of granite 

 ridges" observed in the mines to the north of Cam Brea, running 

 parallel to that hill, are really produced by the subsidence of 

 wedge-like prisms of ground occasioned largely by junction faults, 

 and not to any original ridged surface possessed by the general 

 granite mass. Fig. 145, from H. 0. Salmon,f illustrates what he 

 supposed to be the form of the granite surface in the neigh- 

 bourhood of Cam Brea, East Pool, and Wheal Agar Mines 

 where overlain by killas, while fig. 14a is my interpretation of 

 the self-same facts. In each case it must be borne in mind that 

 there is necessarily speculation, since the junction of granite and 

 kiUas is only visible at a few points, where the lodes and cross- 

 cuts intersect it, the junction at all intermediate points being 

 matter for inference. If my interpretation be correct, the 

 supposed granitic ridges north of Cam Brea, are merely 

 longitudinally faulted junctions of granite and killas. 



These faulted junctions are more common than has been 

 supposed, yet they are by no means universal, as is proved by 

 the numerous instances where granite veins are seen to penetrate 

 the stratified rocks without being cut off, as at St. Michael's 

 Mount, Wicca Cove, and Tremearne. 



Cumulative effects of faulting. It has been observed that the 

 really great faults in the earth's crust, such as the Pennine fault 

 in the North of England, rarely occur in regions which contain 

 many small faults, that they are great in fact because the move- 

 ments of a great length of time have been concentrated in them. 

 In the West of England the amount of faulting at any particular 

 point is rarely very great. With the exception perhaps of some 

 of the junction faults which bound the granite bosses, and a few 

 of those which are known as "great cross-courses," it seems 



* See Foster, " on the great Flat Lode." Quart. Jour. Geol. Soc, 1878. 

 f Mining and Smelting Magazine, Vol, 1, 1862, 



