140 FIELD COLUMBIAN MustuM—GEOLoGy, VOL. I. 
As in previous instances, read andesite or trachyte for syenite. 
This specimen is not in the collections. 
‘‘tor. Phonolite from the San Miguel mine.” 
‘¢yo2. Quartz ore from Juan Sanchez mine.” 
‘*103. [E 1063.] Decomposed ore from the San Miguel mine.” 
A very yellow brecciated quartz with ochreous limonite. 
‘¢104. Decomposed ore from the San Miguel mine.” 
‘¢105. Decomposed ore from the San Miguel mine.” 
‘¢t06. Decomposed ore from the San Miguel mine.”’ 
‘‘t07. Pyrite, ore from the San Antonio mine.”’ 
‘«yo8. Pyrite, ore from the San Antonio mine.” 
‘‘tog. Colorado ore from the San Antonio mine.” 
‘110. [E 1064.] Colorado ore from the San Antonio mine.” 
This is a rusty quartz with pyrite. 
The pyrite, while retaining its crystalline form, is filled with 
straight cracks, for the most part microscopic, which follow octa- 
hedral planes, and are filled with quartz. 
‘«t11. Colorado ore from the San Antonio mine.”’ 
‘‘712. [E 1065.] Gneiss from near Santa Rosa de Osos.” 
This is a black, very schistose rock. Under the microscope 
lt appears as a rock, composed prn cipally of hornblende, with some 
quartz, and bands of a feldspar, which appears as a microperthitic 
intergrowth of albite and orthoclase. There are also a few crystals 
of microcline, and of a feldspar which shows the plagioclase twinning. 
GOLD MINING DISTRICT OF SAN PEDRO. 
About twenty-five miles south of Santa Rosa is Medellin, the 
capital of Antioquia. It is a place of 30,000 inhabitants, on the 
bank of the Medellin River, a branch of the Porce, and at the foot 
of the Santa Elena Mountain, a peak of the range upon the flanks 
of which Santa Rosa lies. From San Pedro, a village near Medellin, 
Sefior Gamba collected three specimens of gold ores. These ores 
are not in the Museum collections, and there is no information 
available concerning this district. 


