
ORE-FORMATIONS 251 
eruptives of great age, simply on account of the extreme denudation 
which those regions have experienced. The upper parts of the older 
lodes, which may have carried quicksilver, have long since been removed, 
along with the country-rock traversed by them, so that it is only the 
pyritic or deep-seated ore-formations which are now encountered in the 
lodes of profoundly denuded regions. Again, Professor Vogt has pointed 
out some remarkable differences between gold-, silver-, and lead-bearing 
veins of relatively recent age, such as those of Comstock, Potosi, 
Hungary, etc., and the much older lead-silver veins of Norway, Bohemia, 
the Erzgebirge, etc. In both cases the lodes are closely associated with 
eruptive rocks, and the country-rock has undergone much alteration, so 
that the conditions attending the deposition of the ores and veinstones 
in all the regions referred to appear to have been similar. The differences 
referred to by Professor Vogt have reference not only to the contents of 
the lodes, but to the changes which have been superinduced on the 
country-rocks ; and these differences, according to him, indicate that 
the older have been formed at a greater depth than the younger veins. 
From his point of view, therefore, the latter, if they were followed down- 
wards, would gradually assume the character of the former. Whether 
such would prove to be the case is, of course, conjectural, but Vogt’s 
hypothesis is to some extent supported by the phenomena revealed in 
certain deep mines. In the Cornish mines, for example, after passing 
down through their gossans the lodes were found to carry copper-ore 
with some tin-stone ; at a still greater depth, a zone of mixed tin-stone 
and copper-ore was encountered, and under that tin-stone almost 
exclusively. So, again, in lodes carrying silver-lead-zinc ores it has 
frequently been observed that the proportion of zinc-blende increases 
with the depth. It would seem, also, that in many manganese-iron 
formations the proportion of iron similarly increases downwards. But 
much additional observation and study will be required before the laws 
governing the genesis and deposition of ore-formations can be clearly 
comprehended. 
Walls of Lodes.—Occasionally the walls of lodes are 
more or less slickensided—owing, doubtless, to the one being 
ground against the other. [The smoothed and slickensided 
stones which not infrequently occur in the contents of lodes 
have already been mentioned, These are probably in some 
cases fragments detached from the walls after the latter had 
been smoothed ; in other cases, they may have been slicken- 
sided zz sz¢u, the blocks and stones being pressed and rubbed 
against each other during movements of the country-rock.] 
Frequently the walls of a lode are more or less decomposed, 
the width of decomposed rock being very variable. Some- 
times it may hardly exceed an inch or two, while in other 
cases the rock may be rotted for many feet or yards away 
