COMPOSITION OF VEIN MATBEIALS. 559 







proceeded much farther. In the latter the oxides of iron and manganese 

 have increased only slightly in amount, and the decrease in carbonates is 

 mainly supplied by the increase in silica, an increase which, as in the 

 former case, must be mainly accounted for as coming from an extraneous 

 source; it was probably taken up by the waters in their passage through 

 the porphyry. 



Nos. 9, 10, and 11 represent the most silicious forms of vein material, 

 comparatively free from bases. Nos. 9 and 10 are the black cherts so com- 

 mon in the ore bodies, and 11 the granular quartz which frequently re- 

 places it, especially on Carbonate Hill. In each the silica is still partly 

 soluble in a moderately strong solution of potash. Besides silica the two 

 former contain iron and alumina and probably a little organic coloring 

 matter. These cherts are thoroughly compact and generally form barren 

 streaks or floors in the ore bodies; sometimes, however, chloride of silver 

 is found coating their cleavage surfaces. The granular quartz, on the other- 

 hand, which is very porous, frequently contains crystalline cerussite partly 

 filling the minute cavities and then constitutes an ore, though, as in the 

 present case, it is liable to be mistaken in the hand specimen for a white 

 quartzite. 



No. 12 is the breccia of White Porphyry in the small ore-body which 

 was found above the Blue Limestone horizon, just west of the small dike 

 in the Evening Star ground. The percentage of silica is above the normal, 

 showing that the ore-bearing waters have removed a portion of the bases. 

 In the cementing material galena is altered partly to pyromorphite, partly 

 to cerussite, and the calcite may indicate that the ore-bearing solutions 

 reached here after passing along the limestone contact or may simply result 

 from the decomposition of the porphyry. 



The assays of vein material and limestones in Table XIV, Appendix B,. 

 show that all the specimens tested carry silver in appreciable amounts, though 

 in the case of limestones which were gathered in the various mines, and in 

 that of the Breece Iron ore, the tenor in silver is much less than is generally 

 credited to those used as flux by the smelters. They are such amounts as 

 might be expected to have been carried into them by the surface water* 

 after leaving the ore bodies. 



