1835. RELATION OF BORNITE AND CHALCOCITE— LANEY. 519 



material already deposited, but the chemistry of the process has 

 not been worked out. Where fractures of two periods are present, 

 both are often filled with chalcocite, that in the younger fractures 

 cutting across the veinlets in the older ones. Fractures also occur 

 in the intergrown chalcocite and bornite, and in such instances the 

 secondary veinlets cut across both the primary chalcocite and the 

 bornite. The relation of the two minerals to each other in the case 

 in hand leaves no doubt as to the secondary nature of the chalcocite. 

 This type of chalcocite, as far as observations have extended, is con- 

 fined to the upper portions of the veins and was not found in sections 

 of ore from the deeper mines. It was found, however, in the upper 

 portions of all mines from which sections were examined, and in many 

 instances a single section would show excellent examples of both 

 types of chalcocite. (See pi. 68, fig. 1.) 



The other type of ore is entirely different. Both minerals are 

 present in every section examined, sometimes the bornite predomi- 

 nating, and at others the chalcocite. They are intimately associated 

 with each other, but each has its own definite boundaries, cleavage, 

 and other physical properties, with absolutely no indications that one 

 is secondary to or derived from the other. In a number of sections 

 the chalcocite predominated over the bornite, and in such instances 

 the indications seemed to be that the bornite was the first to crys- 

 tallize. It occurs in irregular areas, sometimes separated and again 

 connected, lying in a larger area of chalcocite. In other instances 

 the two are present in approximately equal amounts, and there is 

 nothing to indicate that one is older than the other. In other occur- 

 rences, as in the ore from the Blue Wing mine, the two minerals are 

 present as small areas or grains and in approximately equal propor- 

 tions. In these sections the appearance is as if a sponge of bornite 

 while growing had been merged with another similar sponge of 

 chalcocite, the association being so intimate and so complex that there 

 is no way of accounting for it except on the basis of contemporaneous 

 deposition (see pi. 67, fig. 2). In the case of the sections in which 

 the bornite appears to have been formed earlier than the chalcocite, 

 it seems as though when the ores were being deposited, the solutions 

 were first saturated, as it were, for bornite, and this mineral began 

 to crystallize out, the iron possibly being the determining factor. 

 This continued until by a reduction of the bornite molecules m the 

 solution, the eutectic point for both bornite and chalcocite was 

 reached, and these two minerals crystallized out simultaneously, and 

 in many places solidified as crystallographic mtergrowths. Ike 

 chalcocite is rather coarsely crystalline, and the etch figures show 

 that the larger areas are made up of numerous interlocking grains, 

 which stand out distinctly and have no definite crystallographic rela- 



