2 
ECONOMIC GEOLOGY 
Rhodesia, has replaced (77. LM.E., xxxi, 1906, p. 85) 
ilmenite. Although the ferro-magnesian minerals probably 
contain small amounts of other metals besides iron and 
manganese, it is only where igneous rocks have undergone 
strong secondary changes that the less common metals occur 
in workable quantities, These oresare secondary constituents 
of the rock. Dr. A. Brammall gives reference to literature 
in favour of the primary occurrence of gold in igneous 
rocks; but in the two cases he describes the gold is found 
with vein-quartz and such secondary. minerals as tourmaline 
(Miner. Mag., xxi, 1926, pp. 15-16). 
ORE FORMATION IN RELATION To IGNEOUS Rocks—The 
discovery of many metals in igneous rocks and of these rocks 
in most important ore-fields led to the lateral secretion 
theory giving place to the view that the metals in most 
lodes are derived from particles present as original constitu- 
ents of igneous rocks. This dependence of ores upon igneous 
rocks was widely adopted after a paper by the late J. F. 
Kemp on *“ The Role of Igneous Rocks in the Formation 
of Veins” (Tr. Amer. {.M.E., xxxi, 1901, pp. 169-08) which 
is one of the classics of mining geology. Igneous rocks are 
undoubtedly the source of the iron and manganese in many 
ores. The theory was extended to the ordinary lode metals— 
zold, platinum, silver, copper, tin, lead, zinc, nickel, cobalt, 
stc., of which igneous rocks are either barren, or contain 
only minute traces that may be secondary. There is no 
a priori reason why copper should not be a primary con- 
stituent of ferro-magnesian minerals; but as among igneous 
rocks it is most often found in diabase, using that term in its 
English sense, it occurs in altered rather than in fresh rock. 
Gold is found in diorite and porphyrite, where the rock has 
been altered to propylite or by the development of chlorite. 
That unaltered igneous rocks are barren of the ordinary 
metals is indicated by the vast areas of those rocks that 
contain no lodes. The interior of granite masses are generally 
barren in all parts of the world. Scotland includes igneous 
tocks of all kinds and ages, and they have been exceptionally 
closely examined. Quartz-veins in them are innumerable ; 
but lodes are scarce, and the most important in Scotland 
are not in the vicinity of great igneous intrusions ; thus the 
lodes at Wanlockhead are in sediments and the few igneous