302 GEOLOGY OF THE EUREKA DISTRICT. 



These workings, although they do not offer a continuous exposure, are 

 sufficient to give the course of the quartzite all the way from the Jackson 

 to the Albion. Beginning with the Jackson mine, the plane of contact has 

 a course a little east of north, gradually turning more and more to the west 

 until at the Albion it curves slightly south of west. In the lower levels of 

 the mines this contact plane, as mapped by the underground surveys, pre- 

 sents roughly a concave outline curving outward toward the north or away 

 from the granite mass of Mineral Hill. This curve, however, is by no means 

 symmetrical, the tendency of the quartzite in making so sharp a bend 

 being to break abruptly and irregularly and for short distances to be forced 

 outward, assuming directions quite at variance with the general course ; 

 the dip frequently changing with the strike. This tendency of the quartz- 

 ite in fracturing to be forced outward beyond the line of the curve is well 

 shown just west of what is known as the compromise line between the 

 Eureka and Richmond mines. It may be seen all the way from the sur- 

 face down to the ninth level of the Richmond. Mr. Curtis has carefully 

 mapped the underground contacts, not only between the quartzite and lime- 

 stone but for all three formations. By reference to his map 1 both the con- 

 cave outline of the beds and the irregularities of strike and dip may be 

 seen at a glance. 



The overlying limestone and shale conform in their general outline 

 with the quartzite, the shale, however, exhibiting a decided tendency, as is 

 usually the case with argillaceous strata, to bend and fold rather than to 

 break abruptly. 



Across the limestone on the east slope of Ruby Hill runs a profound 

 fault which, on account of its bearing upon the geology of the region, has 

 been designated the Ruby Hill fault. It is a continuation of a line of 

 faulting coming up from the southwest. From New York Canyon, where 

 the Ruby Hill fault leaves the Hoosac fault, to its intersection with the 

 Jackson fault the nearly straight course is easily followed by a chain of 

 rhyolite outbursts as well as by the conformity of strata Where it 

 crosses the Jackson fault its direction is somewhat disturbed and is not so 

 readily made out, but near the American shaft it reappears, with a course a 



'Op. Cit., PI. ni. 



