140 GEOLOGY OF THE EUEEKA DISTRICT. 



Peak quartzites, paleontology again supports structural evidence, the organic 

 forms being such as are only found in the upper horizons or mingled with 

 those having a wide vertical range. 



Spanish Mountain.— Tliis hroad, clcvated niass of Eureka quartzite, nearly 

 two and one-half miles in width, lies due west of Prospect Peak. Its struc- 

 tural features differ from those of any other area of the Eureka Mountains, 

 but at the same time bear some resemblance to those of Grays Peak, both 

 being formed of strata of the same geological age, with the Lone Mountain 

 beds resting upon their slopes. On Spanish Mountain the quartzites dip 

 away in every direction from the summit, but without any clearly defined 

 lines of bedding, presenting the appearance of a great dome-shaped body 

 falling away on all sides. This quartzite is fractured by local displace- 

 ments, but they fail to bring to the surface anj- underlying Pogonip beds, 

 and the few drainage channels, which have cut one or two narrow gorges, 

 still lie wholly within the quartzite. Over this dome-shaped body the Lone 

 Mduntain beds undoubtedly passed at one time; erosion, however, has 

 woi-n them off the summit, with the exception of two small patches, which 

 are sufficient to establish the fact that the upper members of the quartzite 

 are still in place on the top of the mountain. Surrounding the quartzite 

 on all sides occurs the Lone Mountain limestone, except along Spring Valley, 

 where it is probably obscured by recent accumulations. 



Isolated patches of limestone in the valley confirm the opinion that 

 the Lone Mountain beds extend down to the Spring Valley fault. These 

 limestones cross the divide connecting Spanish Mountain with Swiss 

 Mountain and come within 200 feet of the summit of the former. Wherever 

 observed, the limestones rest unconformably upon the quartzite, but, as they 

 are for the most part devoid of bedding plane, no determination can be 

 made of their thickness. Moreover, the line between the Silurian and 

 Devonian is arbitrarily drawn and rests, as elsewhere in tlie district, on 

 lithological distinctions and the absence of evidence of life in the lower 

 rocks. As shown on the map, the thickness ascribed to the Lone Mountain 

 beds varies greatly at different localities, but there is no doubt that the 

 vertical distance between Eureka quartzite and limestone characterized 

 by a Devonian fauna actually does exhibit great variations in tliickness. 



