108 EXPLANATION OF PLATE 67. 



the manner in which the Granite and Slate neai 

 Redruth are intersected by metalliferous Veinsj, 

 terminated abruptly at the surface, and descending 

 to an unknown depth ; these Veins are usually most 

 productive near the junction of the Granite with the 

 Slate, and where one Vein intersects another. The 

 mean direction of the greatest number of them 

 nearly from E. N. E. to W. S. W. They are intern 

 sected nearly at right angles by other and less nu- 

 merous Veins called Cross Courses, the contents 

 of which usually differ from those of the E. and 

 W. veins, and are seldom metalliferous. 



The Granite and Killas and other rocks which 

 intersect them, e. g. Dykes and intruded masses of 

 more recent Granite, and of various kinds of por- 

 phyritic rocks called Elvans (see PI. 1, a 9. h. c.) 

 are considered to have occupied their present rela- 

 tive positions, before the origin of the fissures, 

 which form the metalliferous Veins, that intersect 

 them all. (SeeV. I. p. 550.)* 



* In Vol. I. p. 552, Note, a reference is made to some important 

 observations by Mr. R. W. Fox on the Electro-magnetic actions 

 which are now going on in the mines of Cornwall, as being likely to 

 throw important liglit on the manner in which the ores have been 

 introduced to metallic veins. 



The following observations by the same gentleman in a recent 

 communication to the Geological Society of London, (April, 1836,) 

 appear to contain the rudiments of a Theory, which, when maturely 

 developed, promises to oifer a solution of this difficult and complex 

 Problem. 



" If it be admitted that fissures may have been produced by changes 

 in the temperature of the earth, there can be little difficulty in also 

 admitting that electricity may have powerfully influenced the existing 

 arrangement of the contents of mineral veins. How are we other- 

 wise to account for the relative position of veins of different kinds 

 with respect to each other, and likewise of their contents in reference 

 to the rocks which they traverse, and many other phenomena ob- 



