Mar. 1899. THe Ores or CoLtomp1a—NICHOLs. 131 
meanings they usually carry. From a comparison of the specimens ~ 
in the collection with the names in the catalogue, it appears that the 
rocks mentioned above really include but a few species. Gneiss, 
diorite, mica schist and limestone are found in a few mines. All the 
other rocks mentioned are really andesites, dacites, trachytes, rhyo- 
lites and their tuffs, with small quantities of some basic lava, possibly 
a basalt. 
Antioquia produces much gold and little silver. The gold fol- 
lows the pyrite and the silver the blende. The ores become more 
argentiferous southward, and the content in zinc increases at the 
same time. The only true silver mines are in the Titiribf district in 
the south. 
The deposits may be separated into three groups: The North 
Central, the Western, and the Southern deposits. 
THe NorTH CENTRAL Deposits. These are the deposits of the 
Nechi and Porce valleys and the Central Cordillera. The specimens 
indicate that the deposits are quartz veins of the familiar type. This 
is confirmed by Whitney.* In the unoxidized ores the sulphide is 
pyrite, with which are mingled very small quantities of galena and 
blende. In some districts arsenopyrite replaces the pyrite. The 
North Central deposits include the districts of Remedios, Amalfi, 
Santa Rosa de Osos, Anori and San Pedro. 
Twe WESTERN Deposits. These are upon the west side of the 
Western Cordillera and upon the slopes of the Atrato Valley. The 
nature of these deposits I could not definitely ascertain, but they 
appear to be quartz veins of the ordinary type. Although these veins 
are auriferous, there is considerable calcite in the gangue, and in one 
of the districts there is the unusual case of an important gold deposit 
in limestone. The ores in the unoxidized portions are pyrite and 
arsenopyrite, associated with more blende and gaiena than is the case 
in the North Central districts. Chalcopyrite is also present, and in 
places the ores assume a distinctly coppery character. Tellurides of 
gold are ‘also found in the mines of this region. The western 
deposits include those of Anza and Frontino. , 
THE SOUTHERN Deposits. ‘These ate very different in character 
from those already mentioned. The ores are chiefly blende, mingled 
with pyrite and galena. Antimony replaces the arsenic of the more 
northern districts. The specimens indicate that the deposits are, in 
part, auriferous silver ores in quartz, and, in part, argentiferous gold | 
ores in calcite and magnesite. The quartzose ores appear to be 
*Mineral Wealth of the United States, p. 109. 
