Tunnel of the Tavistock Canal. 151 



It does not often happen that the processes of the miner lead to 

 so much geological discovery as might be expected ; the works he 

 undertakes follow the course of the vein he is exploring, or are 

 confined within a small space bordering upon it. As the veins are 

 sought after in but very few varieties of rock, so the number that 

 are laid open to view is generally limited compared with those that 

 exist in mining districts. 



A tunnel of such an extent as the one now to be described, in 

 such a district, crossing the direction of the metalliferous veins, and 

 passing at such a depth under the surface, could hardly fail of 

 proving an interesting object to the geologist as well as the miner. 



Two facts have been ascertained by its progress : 



1st, Relative to the rocks, that the killas of which the hill is 

 mainly formed, is traversed by beds of other rock, whose direction 

 is inclined to that of the metalliferous veins, and which have a 

 pretty uniform dip or underlay to the north. 



2d, Relative to the metallic veins or lodes, that they traverse all 

 the strata, and that they have a remarkable difference in their dip 

 or underlay on the two sides of the hill. Those on the north side 

 dipping to the north, and those on the south side to the south.* 



Commencing at the north end of the tunnel, I shall proceed to 

 detail the strata that have been passed through, referring to the sec- 

 tion accompanying this paper to shew their position, and to the 

 specimens of the rocks themselves which I have selected to exhibit 

 their character. 



* Since the paper was written it has been ascertained, as I have been informed, that 

 some veins lately discovered in the space between Renfrews shaft and Brays shaft underlie 

 to the south, which is an exception to the preceding observation. But it may be observed 

 that this deviation takes place near the centre of the hill. 



u 2 



