212 GEOLOGY OF THE EUEEKA MSTEICT. 



construction of sections must necessarily, to a great extent, be based upon 

 theoretical reasoning. 



On the geological maps, lines of cross-sections are laid down by nar- 

 row black lines designated at the borders of tlie sheet by lilock letters. 



Section A-B.— This scction is drawn only halfway across tlie Eureka 

 Mountains, and is confined to the northeast corner of the District, atlas 

 sheet VIII. At the extreme western end of the map the section exhibits 200 

 or 300 feet of the Hamburg limestone just west of the Jackson fault, fol- 

 lowed on the east side of the fault b^- the Pogonip limestone, which in turn 

 is capped by Eureka quartzite shown on the summit of Caribou Hill. On 

 the east slope of Caribou Hill the rliyolitic ashes and tuifs conceal the 

 quartzite and stretching eastward across the valley rest against the steep 

 wall of Richmond Mountain. These ashes and tuffs underlie the town of 

 Eureka, although nowhere of any great thickness, and probably over much 

 of this area overlie solid rhyolite not far below the surface. In the section 

 the Eureka quartzite is repi-esented as underlying the vallej' as far eastward 

 as the Hoosac fault or the prolongation of the fault as recognized south of 

 the Richmond Smelting Works. The representation of a narrow strip of 

 Lower Coal-measure limestone is a theoretical deduction based upon strong 

 evidences observed at the latter locality. The basic andesites of Richmond 

 Mountain stretch eastward for 15,000 feet, followed by 7,000 feet of basalts, 

 completely cutting off all evidences of any continuation of either the Spring 

 Mountain or County Peak blocks. The depression of Hunter's Creek and 

 the o-entle inclination of the basalt table toward it are well brought out in 

 cross-section. Nowhere are two epochs of Carboniferous limestone with 

 the intervening Weber conglomerate better shown tliixn along tlie line of the 

 section. Rising above the basalt the Upper Coal-measures exhibit 500 feet 

 of beds dipping 45° to the west, resting upon Weber conglomerate. The 

 conglomerate is inclined at the same angle, but soon develops into an anti- 

 cline standing at 50° east, followed by a syncline varying from 50° east to 

 30° west. It is an admirable example of the effects of lateral compression. 

 The thickness obtained for the complete series of conglomerates measures 

 2,000 feet. Underlying the conglomerates come the Lower Coal-measui-es, 

 the section crossing Alpha Ridge just south of Fusilina Peak and showing 



