520 PROCEEDINGS OF THE NATIONAL MUSEIM. vol. 40. 



tion to each other. The cleavage, as brought out by the etching, is 

 apparently in two directions at right angles to each other, one more 

 prominent than the other, one possibly prismatic and the other basal. 



The crystallographic intergrowths are the most interesting and 

 also the most conclusive as to the contemporaneous deposition of the 

 two minerals. These are by no means rare, having been found more 

 or less perfectly developed in ore from all the mines except the 

 Seaboard. At a magnification of 40 diameters these areas resemble 

 very closely the intergrowths of quartz and feldspar in a micropeg- 

 matite. At the highest magnification used, 220 diameters, this 

 resemblance is even more pronounced. In these intergrowths the 

 minerals present perfectly sharp and clear-cut boundaries, with 

 absolutely no indication of gradation of one into the other — bound- 

 aries just as sharp as between any minerals in an igneous rock (see 

 pi. 68). When an area of such intergrowth was etched deeply 

 enough to bring out the two cleavages distinctly (pi. G9, fig. 1) the 

 chalcocite proved to be a single grain or crystal, the cleavage lines 

 of which could be seen extending from one side of the grain to the 

 other, interrupted here and there by the filaments of bornite. This 

 type of texture is regarded as proof that the minerals crystallized at 

 the eutectic point of a solution, and it is, therefore, conclusive evi- 

 dence that in the case in hand bornite and chalcocite were deposited 

 contemporaneously. 



It is realized that while these minerals are contemporaneous, they 

 both may be secondary after some leaner copper mineral. There are 

 certain reasons for suspecting such conditions, the most prominent 

 of which is probably the long period of erosion which the region has 

 undergone since the ore deposits were formed. This long erosional 

 interval would afford time for conditions of oxidation and enrichment 

 to penetrate to exceptional depths in the ore bodies. With this idea 

 in mind careful observations were made as to the depth of the zone 

 of alteration as far as the same could be determined, and the conclu- 

 sion is that it rarely if ever extends below 175 or 200 feet, the im- 

 permeability of the veins limiting the downward circulation. They 

 and their walls are all exceedingly dense and impervious to water, 

 and the mines all furnish a surprisingly small amount of water, of 

 which by far the greater part comes from the upper 100 feet of the 

 vein. As an example of the tightness of the vein, it may be men- 

 tioned that when the Blue Wing mine was unwatered about two 

 years ago, it was found that the air pressure had held the water 

 out of an upraise which had been started from the 266 feet level. 

 The vein rocks were so tight that the air could not escape even though 

 it was under a pressure of about eight atmospheres. Under such 

 conditions as these, circulation of meteoric waters must necessarily 

 be at a minimum. This tightness of the vein is characteristic of 





