Mar. 1899. THE ORES or CoLompia—NICHOLs. 149 
in all impregnations noticed among the ores of Titiribi and in 
other similar deposits of Colombia, the pyrite precedes the blende and 
appears first as small crystals scattered through the rock. Where the 
alteration has proceeded far, however, the blende usually much 
exceeds the pyrite in quantity. There frequently appear two genera- 
tions of blende; one, small in quantity, is earlier than the major part 
of the pyrite and appears as sharp, well defined crystals; the other, 
forming the larger part of the blende, is in the usual semi-massive 
forms and irregularly bounded crystals overlying the pyrite and 
mingled with it. The few antimonial and arsenical minerals appear 
to be the latest of all. Jamesonite occurs in open cavities coating 
the sulphides. Arsenopyrite, which in some of the more northern 
districts is the principal ore, here occurs only asa druse of needle-like 
crystals coating pyrite. 
It is well known that the minerals of the lavas, especially the 
basic ferromagnesian silicates, are unstable, and when exposed to the 
atmosphere alteration sets in almost immediately. When the minerals 
are broken up and deposited in the form of a porous tuff, this alteration 
is much favored and goes on with great rapidity. The decomposition 
of the basic lime-bearing feldspars and the ferro-magnesian silicates 
sets free the carbonates of lime and magnesia in quantities which are 
at times very considerable. In consequence of this the gangue 
mineral, which in the neighboring Zancudo mines is quartz, is in the 
deposits in tuff, dolomite. As the blende and pyrite were deposited 
contemporaneously with the dolomite, it is probable that these 
minerals also are derived from the decomposition products of the 
country rock, although it by no means necessarily follows. This 
deposit seems to be an example of a valuable deposit formed by a 
leaching of the rocks of the immediate vicinity. A further study in 
the field would be of much interest. } 
From the three mines not included in the Zancudo establishment 
Sefior Gamba collected the following 19 specimens : 
‘195. Sandy material, from the Otramina mine.’’ 
‘196. Veinstuff from the Otramina mine showing the thickness 
of the vein.”’ 
‘197. [E 1081.] Veinstuff showing the thickness of the vein.’ 
This is a half-inch vein of dolomite carrying blende and pyrites in 
a mineralized tuff which is thoroughly impregnated with sand-like 
grains of pyrite. 
“198. Sandstone, wall rock in the Otramina mine.”’ 
Probably this is a volcanic ‘‘sandstone,” ¢. ¢., a tuff. 
“199. Veinstuff from the Otramina mine.” 
