1224 A TREATISE ON METAMORPHISM. 



level in an equally strange and apparently inexplicable manner. The ore 

 masses of exceptional richness are generally called ore shoots. Sometimes 

 they are spoken of as pay streaks, at other times as bonanzas, at other times 

 as chimneys. In this paper ore 1 shoot is used as a general term to include 

 all deposits of exceptional richness or 'size, of whatever origin. At various 

 places in this paper factors have been mentioned which produce ore shoots. 

 However, because of the very great economic importance of ore shoots, it 

 seems to me advisable to consider under one heading some of the more 

 prominent features of ore shoots deposited from aqueous solutions, even at 

 the risk of repetition. 



Ore shootis may be grouped into those which are largely due to 

 structural features, to the influence of the wall rocks, and to secondary 

 concentration by descending waters. 



One large class of ore shoots may 1 be explained principally by structural 

 features. These structural features may be the varying size of openings, 

 varying complexity of fractured, "flexures, intersections of fractures, and 

 later orogenic movements. 



A fracture through a. mask of rocks is necessarily uneven. Where 

 there are movements, it follows that the walls are not adjusted to each 

 other. Where convex surfaces are opposite each other, a fissure may be 

 represented by a mere seam. Where, on the other hand, two concave sur- 

 faces are opposite each other, a widening may occur which in some cases 

 is sufficient to produce a great room. This is beautifully illustrated by 

 the Comstock lode. This lode showed remarkable variations in its width, 

 here being represented by a narrow seam of clay, there by a great 

 bonanza. The story that tells how Fair and Mackay, with persistent 

 courage, drove along a little seam of clay, at places almost unnoticeable, 

 for hundreds of meters, from the 360-meter level of the Gould and Curry 

 mine to the great bonanza, is one of the romances of mining. a The clay 

 seam marked a place where two protuberances of the walls had been 

 jammed together so that there was no opening through which circulating 

 waters could pass, whereas the great bonanza represented a place where 

 both walls bowed outward and thus gave ample space for the deposition 

 of the great deposit. Rooms may be produced also partly or largely by 

 solution, and may be connected by comparatively large channels. 



"Lord, Eliot, Comstock mining and miners: Mon. U. S. Geol. Survey, vol. 4, 1883, pp. 309-314. 



