THE MINING DISTRICTS OF CORNWALL. 339 



otherwise, of the constituents either of one or of any number of 

 such slates. 



In the first place, a great deficiency of silica will be observed ; and, 

 secondly, slates contain, almost without exception, a larger amount 

 of soda than of potash, while the granites and elvans are potassic 

 rocks. 



Mineral Deposits. 



The formation of mineral veins is a difficult and very comprehen- 

 sive subject, and one of which our knowledge is exceedingly limited. 

 I shall therefore, in the present paper, confine myself first to an 

 enunciation of such facts as establish a general connexion between 

 mineral veins and some of the rocks previously described, and sub- 

 sequently offer certain suggestions relative to the probable influence 

 of eruptive rocks on the production of metalliferous deposits. 



The veins producing ores of tin and copper, which in Cornwall are 

 distinguished by the name of " lodes," have a general direction 

 approaching to east and west, and occur in both granite and killas. 

 They are, however, seldom met with at any considerable distance from 

 the junction of these two rocks, or from the vicinity of elvan- courses, 

 which are themselves most numerous in the neighbourhood of the 

 principal granitic areas. These lodes extend without interruption 

 through every rock of the metalliferous series ; but their character- 

 istics are locally influenced by the several formations through which 

 they pass. In killas included fragments of clay-slate prevail, while 

 in granite the veinstone contains pieces of that rock ; quartz and 

 capel (a dark siliceous slaty material), however, are abundant consti- 

 tuents of nearly all lodes, whatever may be the nature of the en- 

 closing rock. The elvans traverse the same districts as the metalli- 

 ferous lodes, and, owing to slight differences of direction or dip, 

 are frequently intersected by them, while the cross veins often cut 

 through both indifferently. It is, however, remarkable that although 

 a very large proportion of the lodes are " heaved " or displaced 

 by cross courses, a small number only of elvans are so affected by 

 them. In two instances only does a lode appear to have been 

 displaced by an elvan*. Both lodes and cross veins dip more fre- 

 quently towards the granite than in a contrary direction. 



The granite of Cornwall is believed to be of Post-Carboniferous 

 age ; and it is evident that the elvans which traverse both granite 

 and killas must be posterior to the consolidation of at least the upper 

 portions of the former. When, however, elvan dykes are found in 

 slate only, their age becomes more doubtful, since some may have 

 been formed before, and others after the intrusion of those passing 

 alike through granite and killas. From their uniformity of compo- 

 sition, their close approach to parallelism, and their general relation 

 to the principal granitic centres, it may, however, be assumed that 

 they are approximately of the same geological age, and are the result 



* This apparent displacement of two tin lodes by an elvan-course occurs 

 at Polgooth, near St. Austell, and is described by my grandfather in a letter to 

 Mr. Hawkins, Cornwall Geol. Trans, vol. i. pp. 151, 152. 



