596 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox Iv: | 
different ores and veinstones. In the south-west of England, for. 
example, a series of fissures running N. and &., or N.N.W. and 
a 
b 
Fia. 307.—DEcEPTIVE SHIFTING OF A VEIN (B.). 
S.S.E., traverses another series, which runs in a more east and west 
direction (W.S.W. to E.N.E., or W.N.W. to E.S.E.),. The latter (cc, 
dd, Fig. 308) in Cornwall contain the chief copper and tin ores, 

Fic. 308.—GrNERAL Map or Fissures IN THE MINERAL TRACTS OF 
S.W. Encuanp (B.). 
while the cross-courses (b b) contain lead and iron. The east and 
west lodes in the west part of the region were formed before those 
which cross them, for they are shifted, and their contents are broken 
through by the latter. To the east, near Exeter, the east and west 
faults a a are later than the New Red Sandstone, and in Somerset 
than the Lias.’ 
Relation of contents of veins to surrounding rock.—It has 
long been familiar to miners that where a vein traverses various 
kinds of “ country ” it is often richer in ore when crossing or touching 
some rocks than others. In the north of England, for example, the 
galena is always most abundant in the limestone and scarcest in the 
shale, the veins in the Great Limestone (150 feet thick or less) 
having produced as much lead as all the rest of a mass of 2000 feet 
of strata put together. In Cornwall and Devon it has been observed 
* De la Beche, Op. cit. p, 659. 
