1823.] Geological Transactions of Cornwall, Vol. II. 299 



however, to be convinced of this, it is necessary to examine them 

 at every place where they have been discovered ; for so different 

 are their appearance, and attendant circumstances, in different 

 parts, that a very plausible theory made with reference to the 

 veins of one spot only, will be found quite inconsistent with those 

 of another. From the whole, however, the following facts may 

 be collected : 



" 1. They occur only at, or near, the junction of the granite 

 and slate. 



" 2. They are not metalliferous. 



" 3. They have no general direction or position. At Mousehole 

 and Polladan Cove, they run east and west : those at St. Mi- 

 chael's Mount, and some at Porth Just, run west-north-west 

 and east-south-east ; those at Polmear Cove, north and south. 

 Some are quite vertical, as at Portcue, Rosemodris, and Polmear 

 Cove : those at St. Michael's Mount and Mousehole are nearly 

 so : others are inclined at different angles, as at Trewavas Head, 

 Porth Just, and Chycornish Cam ; and others quite horizontal, 

 as at Polmear Cove : at the latter place, indeed, they may be 

 seen in almost all positions. 



" 4. Their direction is usually as straight, and their size as 

 regular, as those of true veins ; but in some cases, they become 

 smaller as their distance from the granite mass increases. 



" 5. Their greatest length has never been ascertained : some at 

 Rosemodris may be traced in the slate nearly 200 feet, and are 

 then lost in the sea. 



" 6. The granite of the veins generally appears different from 

 that of the main body ; it is of much smaller grain : it contains 

 a much larger proportion of quartz, and very little mica ; some- 

 times, indeed, no mica at all. 



" 7. The slate which is contiguous to the veins becomes almost 

 imperceptibly changed from clay-slate to mica-slate, and some- 

 times has even the appearance of gneiss. 



" 8. The slate which is close to the veins is frequently much 

 harder than that which is more distant from them, and its tex- 

 ture is, in general, not so slaty. 



"9. At St. Michael's Mount, and Polmear Cove, the veins 

 may be traced to the granite mass, with which they appear to 

 be in complete union, and to form one body, losing entirely their 

 character as veins. Whether the other veins unite with the 

 granite mass or not, has not been ascertained, as the point of 

 junction is seldom accessible, or even visible. 



" 10. At Carn Silver, one of the veins may be traced from the 

 slate into the granite mass. This is the only instance which I 

 have discovered of a granite vein penetrating both the slate and 

 the granite. 



"11. Some veins (as at Carn Silver, Chycornish Carn, and 

 Pendeen Cove) are closely connected with the slate, and the 

 two bodies appear intimately united, and inseparable : in fact, 



