144 FirLp CoLtumpian MustumM—GEo ocy, VOL. I. 
SILVER MINING DISTRICT OF TITIRIBI. 
The content of the Antioquian ores in silver increases south- 
ward, until at Titiribi the mines opened for gold are now operated 
for silver. 
Titiribi (Sz¢zo Viejo) and Sabalétas, two mining towns near Amaga, 
compose the Titiribf district. The population of Titiribf in 1880 is 
given as g,200, a figure which probably includes the population 
of Sabalétas. The deposits are grouped by Restrepo with those 
of Remedios as vefas de sombre, or horizontal veins; but while the 
specimens from Remedios have all the characters of fissure vein 
material, those from the largest group of mines (the Zancudo 
Establishment) in Titiribi, appear to be impregnations of a brec- 
ciated zone in the trachyte. The country rock of the Zancudo 
mines is given by Gamba in his catalogue as porphyrite, trachyte and 
hornblende schist. No specimens of these rocks are now in the col- 
lection, but portions of the wall rock remain attached to many of the 
specimens. These consist of a light-buff colored compact rock, with 
a well developed flow structure. As might be expected from speci- 
mens coming only a few inches from the ores, they are too much 
altered for determination. Under the microscope they appear as 
confused aggregates of exceedingly minute crystals of secondary min- 
erals, penetrated in every direction by microscopic veinlets of quartz. 
A well defined flow structure is all that remains of the original features 
of the rock beyond a faint impression here and there of a portion of 
the outline of phenocrysts which have altered so as to be indis- 
tinguishable from the ground mass. While this may be an altered 
form of either the porphyrite or the trachyte, from its large develop- 
ment in the Zancudo mine it will be considered here as an altered 
trachyte. One specimen contains a fragment of the hornblende schist, 
which proves to be similar in character to the rock common among 
the Mexican mines. It is there known as chlorite schist, and is gen- 
erally regarded as a metamorphosed andesite. The ores consist of a 
mixture of blende, which is the most abundant sulphide, and pyrite, 
with small quantities of galena, pyrrhotite and chalcopyrite, all 
cemented by quartz. The sulphides form a very large proportion of 
the vein material. This material occurs as an impregnation and 
replacement, partial or complete, of a brecciated zone in the trachyte. 
All stages of the replacement are plainly shown in the specimens. 
The first stage is a breccia, which is cemented by a finely granular 
mixture of sulphides and quartz, while at the same time minute pyrite 
crystals appear in the fragments of trachyte. From this stage the 
