158 Mr Kenwood on the Phenomena of 



wards them ; indeed, I have not met with an instance of the 

 latter. 



It is also one of the most generally recognised facts, that 

 veins of copper or tin ore, are more productive when perpen- 

 dicular than when inclined, and that when a change of dip 

 takes place, it is almost immediately succeeded by an alteration 

 in the contents of the lode. 



I am disposed to suspect that grey (vitreous) copper- ore oc- 

 curs more generally in granite and massive slate-rocks than in 

 the schistose rocks ; and that the copper-pyrites is most abun- 

 dant in the latter. I am, however, aware, that there are many 

 and considerable exceptions. 



It is by no means uncommon to find masses of ore close to 

 cross-courses. 



But the elvan courses, of which I have already spoken, are 

 supposed to play no insignificant part in the economy of the 

 metalliferous districts. They are frequently several fathoms 

 in breadth, and are chiefly composed of a basis of felspar and 

 quartz, containing imbedded crystals of both these minerals, 

 and frequently of many others. As a general rule, they are 

 intersected by the metalliferous veins, cross-courses, &c., but 

 they are seldom heaved; a case, however, of an elvan course 

 heaved by a flucan, occurs at Swan-pool, near Falmouth, and 

 has been well figured and described by Mr Thomas ; whilst at 

 Polgooth the elvans heave some of the lodes. In the vicinity 

 of these veins, many of our mines have been very productive ; 

 of tin, at Polgooth, Wheal Vor, and the Wherry ; and of cop- 

 per, at the Consolidated and United Mines, Ting Tang, Tres- 

 kerby, Dolcoath, Wheal Alfred, Wheal Fortune, &c. At the 

 Battery Rocks here (Penzance), at Swan-pool, before mention- 

 ed, and at St Agnes, the elvans are beautifully shewn on the 

 coasts. 



Having now briefly described the contents of the lodes, and 

 the composition of the elvans, which vary too in their compo- 

 sition, whether they occur in slate or granite, we have to see of 

 what the cross-courses consist. They are generally of quartz, 

 which is often of a peculiar radiated structure, with abundance 

 of clay ; and when the latter prevails, they are called Jlucans. 

 Cross-courses and Jlucans change their character very frequent- 



