GOLD MINING IN COLOEADO. 541 



a]3art; on the south wall there seemed* to be a fissure by itself, only an inch 

 or two wide, and filled with a soft clayey and siliceous material, next to which 

 was a belt of barren rock that might be described as a granulite, or a granite 

 poor in mica; and north of that, next the north wall of the vein, another and 

 wider belt of true vein-matter. Bunches of the latter may be found in places 

 scattered through the "cap." This condition of things may suggest the idea 

 that these veins were originally formed or filled by dikes of granite or granu- 

 lite, and that by a subsequent enlargement or widening of the fissure the 

 siliceous and metal-bearing vein-matter was introduced by other processes of 

 infiltration or segregation by which it is generally believed that fissure veins 

 have been filled. The ore-seams and their gangue are frequently, indeed gen- 

 erally, arranged in layer or banded form, with a considerable degree of paral- 

 lehsm and with the drusy character of true vein filling. Movement in the 

 walls would naturally occasion the irregularity in the width of the crevice, and 

 the shattering or fracturing of the original dike material would afibrd oppor- 

 tunity for the intermixture of the newer vein-matter with the old. Move- 

 ment in the case of the vein now under consideration is clearly evidenced by 

 the " slickensides, " or polished surfaces, already referred to. 



It has already been stated that, where ore-bearing, this vein carries a con- 

 siderable proportion of zincblende and galena. These occur sometimes inti- 

 mately mixed with the iron and copper pyrites, while in some places they are 

 arranged in distinctly separated seams. In the stope east of the east shaft the 

 vein was 12 or 15 inches wide ; on the north wall was a very thin selvage of 

 soft, clayey material, followed by a seam of dark-colored blende and galena, 

 two or three inches wide, somewhat mixed with and succeeded by a seam of 

 greenish, siliceous, and perhaps talcose vein-matter, carrying finely-divided iron 

 pyrites ; then a seam of solid iron pyrites, two or three inches thick ; the 

 remainder of the vein was a quartzose material carrying finely-distributed 

 pyrites suitable for crushing in the stamp mills. In this place there was but 

 little copper present, though elsewhere copper pyrites is sometimes largely 

 represented, and argentiferous gray copper occurs sometimes with the galena 

 and the zincblende. 



Concerning the yield of the rock, on a large working scale, the writer 

 could obtain but little positive information. The stamp-rock, it is said, yields 



