GENERAL CONCLUSIONS. 
The gold and silver ores of Colombia occur either in the acid 
lavas, which have been erupted at intervals from the close of the 
Tertiary to the present time, or in Archean schists in the imme- 
diate vicinity of the lavas. In the schists they are usually poor 
in depth. Owing to the action of the heavy tropical rains, the 
weathered zone of the deposits has often been greatly enriched, 
and it was such enriched deposits that gave the immense yields 
of the early days of Colombian mining. The absence of specimens 
in this collection from such enriched zones, and the general excess 
of sulphide over oxide ores, indicate that the mines are in general 
sunk below this zone. Many of the sulphides in the collection are 
exceedingly rich, and come from deposits that carry large quan- 
tities of the precious metals at depths to which no surface enrich- 
ment could possibly reach. 
The ores of Colombia are of two classes, those occurring in 
quartz in fissure veins of the ordinary type, and those formed by 
segregation from the surrounding rock. The fissure vein deposits 
are those of North-Central Antioquia, and present no feature of 
especial interest. The other deposits, formed by segregation from 
the surrounding lavas, differ in character according to the nature 
of the material in which they occur. In all deposits of the second 
class, the prevailing sulphide is blende. Those in andesite and 
other lavas have a quartz gangue, and are of metasomatic origin. 
Some, if not all, are in brecciated. zones, in which the fragments 
are both replaced by and cemented with the ore-bearing quartz. 
All stages of replacement may be traced in the specimens, The 
ores in the tuffs form the most interesting class. They differ much 
from the above-mentioned deposits, although the two kinds some- 
times occur within a few miles of each other. The tuffs, formed 
of comminuted lavas, decompose very rapidly, especially under the 
influence of the hot, acid waters, which accompany volcanic erup- 
tions and often persist for many years after. The decay of the 
ferro-magnesian and lime-bearing minerals of the tuffs forms, as the 
gangue of these ores, sometimes dolomite and sometimes calcite. 
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