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 
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 
wliich are now going on in the mines of Cornwall, as being likely to 
tlirow important light on the manner in which the ores have been in- 
troduced to metallic veins. 
The follomng 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 whicli they traverse, and many other phenomena ob- 
