
248 STRUCTURAL AND FIELD GEOLOGY 
rock it traverses, as is frequently the case with quartzose lodes, 
it projects at the surface, and forms what miners term a reef 
On the other hand, should a lode be less durable than the 
country-rock, its outcrop is revealed by a trench-like depres- 
sion. Lodes, however, are often concealed underneath super- 
ficial deposits. Some, again, do not reach the surface—either 
owing to the dying-out of the fissures, or to the subsequent 
accumulation above the country-rock of later sedimentary or 
igneous formations. It is probable, indeed, that lodes exist 
in many unsuspected places—more particularly in regions 
where considerable unconformities occur. They are met 
with traversing rocks of all ages—Palzozoic, Mesozoic, and 
Cainozoic alike; but, as might have been expected, are 
of more frequent occurrence in Palzozoic than in Mesozoic, 
and in Mesozoic than in Cainozoic rocks. They are most 
commonly associated with metamorphic rocks and eruptive 
masses, although this is by no means invariably the case. 
Gossans.—A lode at its outcrop is usually more or less 
weathered, and of a rusty brown, red, or yellowish colour from ~ 
the frequent presence of ferruginous matter. Such weathered 
backs are termed Gossans. The thickness or depth of gossans 
is quite indeterminate. Sometimes they extend to a depth 
of many fathoms, but usually they do not go much below the 
water-level of a district. Native metals (gold, silver, copper), 
carbonates, sulphates, and phosphates of metals, and other 
metalliferous compounds often occur in relatively large pro- 
portion in gossans. All these are the products of the decom- 
position of the ores of the original or unaltered lode. As the 
lode is followed to greater depths, native metals and oxidised 
ores gradually disappear, and are succeeded by sulphides or 
other compounds devoid of oxygen. As the present surface 
at which lodes crop out must be far below that which existed 
at the time of their formation, it will be readily understood 
why metals such as gold and silver should often occur in 
relatively large proportion in the gossans of auriferous and 
argentiferous lodes. The outcrop of a vein is necessarily 
lowered with the general lowering of the land-surface by 
denudation. The chemical action of percolating water 
affects the metalliferous contents of the lode, which ere long 
become oxidised, and may even be reduced to the state of 

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