20 
ECONOMIC GEOLOGY 
of porphyry. Where, as at Mt. Lyell, the rocks in contact 
are quartzite and slate, the fissuring and replacement were 
in the slate. This view has been adopted for the Spanish 
field by J. H. Finlayson (Econ. Geol., v, 1910, pp. 357-72, 
and 403-37), and Collins (77. I.M.M., xXxxi, 1922, p. 103); 
and the Skouriotissa Mine in Cyprus has been explained by 
C. G. Cullis and A. B. Edge as due to the replacement of 
pillow-lava (Ming. Mag., xxviii, 1923, p. 342). 
The replacement theory is supported by (1) the absence 
of contact metamorphism or of the baking of the slate; 
(2) the molecular replacement of porphyry or slate by 
pyrites, and not of its displacement by a molten intrusion’; 
(3) the frequent gradual passage from rock to ore, as described 
by Collins and Finlayson, and clearly shown at Rio Tinto; 
(4) the occurrence of the ore bodies in zones of shearing and 
faulting, the association with igneous rocks being due to the 
intrusions having made zones of weakness that were liable 
to subsequent fracture and impregnation by solutions from 
below; (5) the presence in the ore of about 3 per cent. of 
free silica, which would have been converted to iron silicate 
if the ore had been molten ; (6) the presence of such charac- 
teristic hydrothermal minerals as sulphides of iron, copper, 
lead, and zinc, also of gold and silver, quartz and sericite, 
and the absence of tourmaline, apatite, primary micas, 
pyroxenes, and iron-silicates, which are the characteristic 
igneous or pneumatolytic minerals. 
Lenticles of similar nature are well known at Rammels- 
berg in the Harz, at Ducktown in Virginia, and in the lower 
part of the Mt. Morgan gold mine in Queensland (cf. p. 52). 
B. SeconDarRY ORES 
SECONDARY ENRICHMENTS—(a) Burr, Montana ~The 
solubility of copper salts has led to the segregation of the 
disseminated primary copper minerals of contact and sedi- 
mentary ores. These secondary concentrations are the 
mainstay of some fields, as at Butte, Montana, which long 
gave the United States its predominance in copper output. 
Mining began at Butte in 1864 for alluvial gold; silver was 
worked from 1865 to 1893, and copper, unsuccessfully, from 
LR. H. Sales, 77, Amer. LM.E., xlvi, 1911, pp. 3-109.