no. 1835. RELATION OF BORNITE AND GHALCOGITE—LANEY. 5lV 



was followed by a period of quiescence, marking the beginning of the 

 outpouring of the basic lava — the andesite. During this time the 

 regular agencies of erosion were active, and the results were the beds 

 of land-waste intermixed with basic volcanic ash. The vulcanism 

 increased, and there followed the great beds of andesite and andesitic 

 tuff. Finally the activity decreased and a period similar to that at 

 the beginning of the basic activity followed, and during this time the 

 upper beds of sandy tuff were formed. This marked the close of the 

 volcanic activity as far as any record in the rocks goes. Following 

 this the beds were complexly folded, much mashed, and the present 

 schistosity largely developed. This was followed, probably after a 

 long period, by the intrusion, first of the gabbro, later of the granite, 

 and then, as phenomena concomitant with the coming in of the 

 granites, the fracturing of the greenstone schists and the formation 

 of the veins and the development of the ores. 



VEINS AND ORES. 



Veins. — The veins are composed dominantly of quartz with locally 

 a considerable amount of epidote and calcite. (See pi. 63.) In width 

 they vary from small stringers not more than a few inches up to 15 or 

 20 feet. They always have well-defined walls and are probably true 

 fissure veins. As is always the case with such veins, these present 

 many irregularities, most prominent of which are the numerous pinches 

 and swells, both horizontally and vertically. In places they are 

 reduced to little more than a mere stringer of quartz between two well- 

 marked walls, while again they may locally swell out to several times 

 their average thickness. In length they range from a few hundred 

 yards to 4 or even 5 miles, and in many instances may be traced these 

 distances by actual outcrop or by abundant quartz debris in the soil. 

 Vertically they are also continuous, and aside from the irregularities 

 in width they are as well defined in the bottom of the deepest shafts 

 as at the surface. The size of the vein and the prominence of the 

 outcrop form no criteria as to the richness of the mineralization. 

 Often the richest ore bodies have been found under a very insignificant 

 outcrop, and as often the strongest exposure at the surface is barren 

 or very lean. The average strike of the veins is more northerly than 

 the schistosity of rocks in which they occur, and while at times they 

 follow the schistosity for short distances their average strike inter- 

 sects it at acute angles. The fractures in which the veins have formed 

 are therefore regarded as having been made subsequent to the devel- 

 opment of the schistosity in the country rock. The ore is not evenly 

 distributed throughout the veins, but is concentrated locally into 

 definite ore shoots. These present the usual irregularities and as a 

 rule appear to have a slight southerly pitch in the vein. 



