MARYixE.] GEOLOGY FEATURES OF THE ilOUNTAIN BORDER. 131 



just east of the junction of the ISTorth and South Boulder Creeks, to a 

 height of 275 or 300 feet. From this point the trend is very nearly 

 northeast, and for a mile the points of the ridge all reach a height of 

 over 200 feet. In another mile it falls to the plain and disappears. On 

 the south side of the dike abuts a remnant of a gravel terrace. In a 

 direction nearly west of the west end of the main dike, and near Boulder 

 City, a small outcrop again shows itself above the plains. This wall of 

 rock weathers in very square blocks. A specimen shows the lava to be 

 a doleritic basalt composed of a very hard crystalline base, much more 

 crystalline than the golden lava, of handsome dark gray color, in which 

 a greenish transparent feldspar seems to predominate, and containing, 

 porphyritically imbedded, numerous crystals of black augite. North of 

 Valmont some miles, a conical hill would seem to indicate the presence 

 of another small dike. 



There is no reason why lava did not flow through the vertical crevice 

 or fracture which here broke across the sedimentary rocks, eventually 

 giving form to the dike which we now see, and pour out over the then 

 higher surface of the country in sheets, as at the Table Mountains of 

 Golden City, Destroy the flat tops of the latter, as erosion is certain 

 to do in time, and beneath them, at least of the North Table, a/ac simile 

 of the Valmont dike would probably be found. Indeed, above and near 

 Valmont the old lava flows, which poured through the dike and flowed 

 over the surface of the country, were first probably formed into table- 

 mountains and then gradually worn away until erosion has entirely 

 removed all evidences of them save the lava which remains as a dike in 

 the crevice through which the earlier lava flowed. 



GE^^ERAL STRUCTURAL FEATURES OF THE MOUNTAIN-BORDER RE- 

 GION. 



It has been quite impossible in giving, in the preceding pages, the 

 more special features of each formation, not to refer more or less to the 

 manner in which the whole series of sedimentary rocks has been folded 

 at diflerent points along the face of the range. These general structural 

 features, however, have so many points of interest attached to them that 

 they deserve to be considered separately. 



On Plate II are gathered seventeen geological sections across this zone 

 of folded strata. They trend as nearly as possible at right angles to 

 the outcropping beds, or approximately east and west, extend from the 

 archsean Tocks on the west across the more prominently exposed ledges 

 of sedimentary rocks, and are arranged one below the other in their 

 natural order from north to south, No. 1 being farthest north, No. 17 

 farthest south. The map upon the plate covers the northern portion of 

 this zone, and shows the positions of the first nine sections. The char- 

 acteristic topography of the region is given in contours, approximately 

 200 fee^^ apart vertically, and the geology is indicated as far as possible 

 without the aid of colors and without obscuring the topography. Since 

 the outcrops of Cretaceous No. 1 and the border of the archaean rocks 

 here give the key to the general structure, they are indicated more promi- 

 nently than the other formation. The positions of the remaining sec- 

 tions, from 10 to 17 inclusive, are given on the map, showing the distri- 

 bution of coal, (Sec. (Fig. 4.) Sections 1 to 9 are drawn on a scale of 

 three-quarters of a mile, or 3,520 feet to one inch j sections 10 to 14 on 

 a scale of 800 feet to one inch ; sections 15 to 17 on a scale of 1,000 to 

 one inch ; the vertical scale in all cases being as nearly as possible the 

 same as the horizontal. Sections 3 to 9 are arranged exactly north and 



