of Devonshire and Cornivall. 165 



degree of richness, for Instance, if a vein was in grauwacke, it would 

 be impoverished by entering the granite, and vice versa. 



It v/as formerly conceived, that the veins of copper belonged ex- 

 clusively to the grauwacke, and that those of tin were chiefly confined 

 to the granite ; but there are now several copper veins worked in the 

 primitive rock : it is however true, that they are near its junction 

 with the grauwacke.* 



Cross courses are veins of marl or clay which intersect the true 

 veins in Cornwall nearly at right angles to their direction, that is to 

 say, which run from north to south. The most considerable of 

 these cross courses extends from sea to sea ; it passes directly through 

 the meridian of St. Agnes, leaves in the middle of its course the 

 parish of Stythians, three quarters of a mile to the west, and termi- 

 nates on the south in the neighbourhood of Pedn-Boar-Point : it 

 varies from a few inches to some feet in thickness : the depth to 

 which it penetrates is still undetermined. It not only intersects all 

 the true veins, but it has thrown the western portion of those veins 

 some fathoms to the north of the corresponding portion on the east 

 side of it. These cross courses are evidently of posterior formation 



* Mr. Kirwan has endeavoured to explain, why ores are found less frequently in gra- 

 nite than in all other rock formations. He says, " Hence we see why metallic veins sel- 

 " dora occur in granitic mountains or those of jasper, and the harder stones, as their tex- 

 " ture is too close to permit the percolation of water, at least in sufficient plenty, and 

 " because their rifts were previously occupied and filled with stony masses, as being more 

 " soluble, and therefore soonest conveyed into them ; thus silex sufficiently comminuted 

 " is soluble in about one thousand times its weight of water, or even less, whereas raetal- 

 " lie substances require much more; but if the granitic stones are in a state of dccompo- 

 " sition, as in the lower mountains they often are in Cornwall, &c. there they may be 

 *' metalliferous. On the other hand, gneiss and schistose mica, argillaceous porphyry, 

 " and argillites being much softer, are the principal abodes of metallic ores." Geological 

 Essays, p. 412. 



