108 EXPLANATION OF PLATE 67. 



the manner in which the Granite and Slate near 

 Redruth are intersected by metalliferous Veins, 

 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 is 

 nearly from E. N. E. to W. S. W. They are inter- 

 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 in- 

 tersect them, e. g. Dikes and intruded masses of 

 more recent Granite, and of various kinds of por- 

 phyritic rocks called El vans (see PI. \,a 9. b. c.) are 

 considered to have occupied their present relative 

 positions, before the origin of the fissures, which 

 form the metalliferous Veins, that intersect them all. 

 (See V. I. p. 411.*) 



* In Vol. I. P. 413, Note, a reference is made to some important ob- 

 servations 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 light 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 offer 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 positions 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- 



