136 REMARKS ON THE 



In order to form a junction between the Tavistock Canal 

 and the Tamer, it became necessary to drive a tunnel, for 

 a mile and a half, through a hill called Morwel Down, which 

 promises to be a source of interest to the geologist. In 

 forming this tunnel, several powerful veins of clay porphyry 

 have been penetrated, the substance of which is in some places 

 Nos. 16. to 19. much disintegrated, in others firm and compact ; veins suppo- 



sed to correspond have since been observed on the sinface. 

 20. In the tunnel, the Clay Porphyry alternates several times with 

 the Killas, which is here of a light-grey colour, and a soft 

 friable texture. 

 In the course of this undertaking, two workable metallic veins 

 have been intersected, no traces of either had been found on 

 the summit of the hill, although diligently examined. 

 SI. Passing the Tamer, we enter Cornwall, and at Gunnislake, is 

 a mine of Copper in Granite ; and a little beyond, at Drake- 

 walls, there is another of Tin, in Killas. 



22. Tin-vein in Granite, from Carclaze, near St Austle. 



23. Killas found on the road from St Austle to Carclaze ; this I 



consider a very perfect specimen of Grauwacke. 



24. Mass of white Quartz, of which tlie roads are formed. 



25. Mixture of crystallised Quartz and Wolfram, covered with a 



coating of Bitumen, found in Poldice mine near Redruth, 

 at the depth of 106 fathoms, in Granite. 



26. Arsenical Pyrites, mixed with acicular, dark, greenish-grey 



crystals, supposed to be actinolite, from Blaney's Shaft, a 

 branch of Wheal Unity. 



27. Granite, Cairn Brae. 



28. From a vein which traverses the north-east side of Cairn Brae: 



on the spot it appeared to me to be Clay-Porphyry, in hand 

 specimens it resembles fine-grained Granite. 



Vein-stone, 



