


250 STRUCTURAL AND FIELD GEOLOGY 
galena and hematite. Very often, however, the large ore-bodies occur- 
ring in limestone, are simply cases of metasomatic replacement. The 
precipitation of ores, indeed, would seem to have been frequently induced 
by chemical reaction between metalliferous solutions and the country- 
rock. If the latter contained carbonaceous matter, for example, this 
would bring about the deposition of sulphides from solutions of metallic 
sulphates. Precipitation might also be expected to occur in places where 
subterranean currents, differing in temperature and in the nature of their 
solutions, came together. Further, in the case of ascending currents it is 
obvious that gradually diminishing heat and pressure must have played 
a dominant réle in determining the deposition of substances held in 
solution. 
But when we study the succession of minerals in banded lodes, it must 
be admitted that no general law governing that succession can be 
recognised. We can only conjecture that the chemical composition of 
the solutions circulating through the fissures may have varied from time 
to time. Sometimes it would appear as if successive deposition had 
been determined by the relative solubility of the minerals. Frequently, 
for example, quartz lines the walls of a lode, and is overlaid by calcite. 
Again, it is highly probable that the earlier deposits of ore in a lode may 
not infrequently have played the part of precipitants to later introductions. 
Thus copper solutions might be reduced by iron-pyrite, the reaction 
giving rise to the formation of chalcopyrite. It is well known also that 
the iron-pyrite of auriferous quartz-veins frequently contains gold. In 
short, it seems not at all unlikely that many of the common associations 
of ores referred to above may be the result of one ore having acted 
as the precipitant of another. 
Even in one and the same lode the mineral succession is often repeated 
several times, showing that at intervals similar conditions have recurred 
again and again. As the same succession of minerals may appear in 
lodes filled at widely separated geological periods, while lodes of the 
same age may differ greatly as regards their contents and the order of 
mineral succession, it is obvious that the nature and arrangement of the 
ores and other minerals in lodes can tell us nothing as regards the 
geological age of the deposits. 
So far as observations have yet gone, it would seem that differences 
of depth have had considerable influence on the deposition of minerals 
in lodes. In many regions where lodes are worked, the present surface 
of the ground must be several thousand feet or even yards below the 
surface that existed when those fissure-veins were filled. In other cases 
we have no reason to believe that any such excessive denudation has 
taken place. We have the opportunity, therefore, of studying ore- 
deposits which have been formed at very great depths, and comparing 
them with others of much less deep-seated origin. Professor De Launay 
has cited quicksilver-fermations as an example of the latter—since they 
appear to be restricted chiefly to rocks of relatively recent geological age 
which have been traversed by eruptive masses. According to De 
Launay, they do not occur in regions of older rocks or associated with 
