SPANISH MOUNTAIN. 141 



The horiiblende-andesite body on the edge of Dry Lake Valley, at the 

 southwest base of Spanish Mountain, will be discussed in the chapter 

 devoted to igneous rocks, which form a most important group, not only in 

 themselves, but in connection with similar outbursts in Sierra Valley and 

 elsewhere. Here at Dry Lake they present a marvelous variety in color, 

 density and texture, but on careful study they are shown to be closely 

 related, with a marked similarity in mineral and chemical composition. 

 The small body designated on the map as dacite is simply an extreme form 

 of the larger mass, being characterized by considerable free quartz and 

 biotite, and has much the nature of a pumice, while the main body might 

 be designated more concisely as an andesitic pearlite. 



North of Spanish Mountain, as elsewhere, the Lone Mountain lime- 

 stones pass gradually into those of the Nevada epoch, and with this change 

 the structural features of the region assume new aspects, quite different 

 from the rest of Mahogany Hills or Fish Creek Mountains. From Brush 

 Creek northward the structure is that of a simple monoclinal ridge, 

 trending about north 40 west, with a dip invariably to the east. Rising 

 above the Quaternary accumulations along the east base of the ridge in 

 Spring Valley, at sufficiently frequent intervals to prove the continuity of 

 strata, occur exposures of quartzite beds, conformably overlying the lime- 

 stones. As the latter beds bear ample testimony of their Devonian age to 

 the very summit, the siliceous strata have been referred to the Diamond 

 Peak horizon of the Carboniferous. Brush, Modoc, and Signal peaks are 

 the culminating elevations along this limestone ridge, which stretches north- 

 ward all the way to The Gate. Along the west base of these peaks 

 runs the Modoc fault, extending, southward from Hay Ranch Valley, near 

 The Gate, till lost in the Lone Mountain limestones west of Brush Peak. 

 This fault brings up the Diamond Peak horizon in juxtaposition with the 

 Devonian, leaving the limestone ridge between two nearly parallel belts of 

 quartzite of the same age, conformable on the east side, but unconfonuable 

 on the west. As the line of the fault follows the contact between two dis- 

 similar rocks it is easily traced. North of Signal Peak erosion has worn 

 out a deep ravine along the contact, and still farther southward the east 

 drainage of lieilley Creek also owes its origin to erosion along the same 



