

ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 337 
favourable. It is obvious that he must be guided in his 
investigations by the principles already illustrated in con- 
nection with geological map-making. He must be on the 
outlook for every indication that may lead him to the hidden 
treasure. Any marked change in the form of the ground 
will be noted—such as a prominent narrow ridge running in 
a linear direction—or a like well-marked hollow. The former 
may mark the outcrop of a lode of harder consistency than 
the rocks it traverses, while the latter may indicate the back 
of a vein filled with less resistant mineral matter. Even 
when the outcrop of a lode produces no marked surface 
feature, it will yet, in many cases, betray its presence by more 
or less pronounced coloration of the soil. This is due, of 
course, to the decomposition of the minerals occurring in the 
vein. Very frequently the overlying soil will be stained red 
or yellow, owing to the presence of iron-oxides, which are of 
common occurrence in most gossans. Other minerals, if 
sufficiently abundant, will indicate their presence by various 
tints or hues. Green colours, for example, are yielded by 
ores of copper, nickel, or chromium ; commingled blue, green, 
and red stainings are yielded by copper ores; lead-ore is often 
indicated by yellow and green stainings; manganese-ores 
are black, while auriferous quartz is often rusty and cellular 
from the removal of pyrites. Again, a lode may be indicated 
by springs coming to the surface along a definite line, for 
lodes often act as subterranean dams ponding back the water 
that descends towards them along the various division-planes 
of rocks, and forcing it to the surface. Such springs not 
infrequently contain much mineral matter in solution, and 
may give rise to superficial accumulations of tufa, limonite, 
etc. Should the water contain deleterious ingredients (derived, 
for example, from the decomposition of iron pyrite), it will 
naturally produce a more or less striking effect upon the 
vegetation. A pyritous lode is, in this way, sometimes 
indicated by the poverty-stricken character of the vegetation 
in its immediate neighbourhood, which may be in strong 
contrast with that covering the country-rock. 
Fragments of veinstone and ore occurring in a water- 
course will naturally be suggestive, and should the fine gravel 
and sand of the stream contain grains and particles of gold 
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