i8o 
The American Gcolos:ist. 
September, 1902 
pyrite, and the latter mineral is noticeably poor in copper. The 
same general statement also holds true for the porphyry. 
The Dolcoath mine of Cornwall is perhaps the best-known 
example of a vein, the mineral contents of which vary with 
the nature of the enclosing rocks. As described by many 
writers, the veins carry copper-ores in slate and tin-ores in 
granite. Stretch* gives a further example of argentiferous 
galena with its usual associated blende and pyrite in a decom- 
posed plagioclase porphyry, changing to auriferous arseno- 
pyrite in the underlying granite. 
Conni'ali. — Fissures crossing the contact of granite or 
other intrusive rock with sedimentarv rocks are not uncom- 
FiG. 5 
Fig. 6 
Section across Wheal Alfred Gwinear 
Cornwall. Rich ore occurs in the 
Dike only. (From De la Beche.) 
Yam Split in Traversing 
Jointed "Elvan" Dike 
(De la Beche.) 
monly productive, when the district is metalliferous. This is 
very marked in the Cornwall mines, where bunches of ore 
occur at the junction of granite and schist. De la Bechet men- 
tions fissures traversing schists and passing through a dike 
of porphyry (elvan) some 300 ft. thick. The vein above the 
dike carried little ore ; in the elvan, ore was abundant and rich, 
but became poor again in the slates beneath (Fig. 5). This oc- 
currence of ore-bunches where fissure-veins cross such dikes 
has always been known to the Cornish miners. When the lodes 
])ass into the dikes, they are often branched and split, as shown 
in Figs. 3 and 6 (after De la Beche) ; and in such cases, though 
the total amount and richness of ore be the same, the vein 
may not pay to work, on account of the large amount of waste. 
* "Prospecting, Locating and Valuing Mines," p. 135. 
t Geol. Observer, p. 678. 
