Mar. 1899. THE Ores or CoLomBia—NICHOLS. 173 
The veins are very narrow and irregular, one of the most import- 
ant reaching a width of less than two inches. Often in place 
of a single vein there is a zone of branching dolomite or calcite 
threads in the tuff. These features are evidence of a deposit formed 
by a leaching of the surrounding tuff. 
In Tolima many of the ores occur in quartz in mica and chlo- 
ritic schists of Archean age, but always in the vicinity of one of the 
acid lavas. Mr. John C. Randolph says that these veins are narrow, 
that they are always intercalated with the schist and never cut it. 
They pinch out and reappear. Add to this their general tendency to 
grow poor in depth and their association with the lavas which furnish 
the metals for the deposits above mentioned, and it seems probable 
that these deposits are the result of a downward flow of waters which 
have leached the overlying eruptives. It appears that except for the 
vein deposits of North Antioquia, the precious metals, in the districts 
here'considered, came ‘to the surface in the lavas from which they 
vhave been leached. This leaching may have been done by hot sol- 
fataric waters following the eruption, or may have been the result of 
the abundant tropical rains. In either case the deposits would, as 
observed, be richer nearer the surface. This original greater rich- 
ness at the surface when increased by the secondary enrichment of 
the oxidized zone through weathering, as described by several observ- 
ers, accounts for the wonderful richness at the surface found by’the 
original miners. 
