160 DESCRIPTIVE GEOLOGY. 



character and color of the soil, though over the long, smooth slopes, extend- 

 ing to the foot of the Eawlings Peak Ridge, no outcrops were found. 



E,A.WLINGS Peak. — In a direction a little west of north from Rawlings 

 Station extends a double-crested ridge, which represents the remnants of 

 an anticlinal or quaquaversal fold, enclosing a body of Archaean granite. 

 Through the middle of this fold runs a little north and south valley, follow- 

 ing the direction of the main axis, while at right angles to it the southern 

 end of the fold has been cut through, and in part carried away by the erosion 

 of a stream, now dry, which drained the region to the west and north of 

 Separation Peak. There are thus exposed a series of conformable sedi- 

 mentary strata, in which the lowest beds probably correspond to the Prim- 

 ordial sandstones of the Laramie Hills. 



The granite is exposed by the erosion of this longitudinal valley, and 

 is best seen toward its southern end; here, on the east side, its surface, being 

 directly exposed to the west winds, has been beautifully grooved and pol- 

 ished by the desert sands. It shows quite distinct lines of bedding, which 

 hav£ an inclination of 45° to the westward, while the overlying quartzites 

 and sandstones of Rawlings Peak dip 10° to the east. This granite, which 

 has been classed by Professor Zirkel as a granite-gneiss, is a compact, 

 greenish, rather fine-grained rock, which, although wanting in the distinctly 

 crystalline structure of an eruptive granite, has none of the laminated 

 structure of a gneiss. It is made up of quartz, hornblende, and feldspars, 

 with a decided predominance in the latter of plagioclase over orthoclase. 

 Its greenish color is due to the prevalence of finelj^-disseminated horn- 

 blende through the mass. Under the microscope, both quartz and feld- 

 spars are seen to contain a great number of liquid-inclusions, which, partic- 

 ularly in the quartz, are remarkable for containing most perfectly-formed 

 cubes of salt, a hitherto unusual occurrence in the quartz of Archaean 

 granites. There are also observed in the liquid-inclusions of the quartz, 

 besides the cubes of salt, green hornblende-microlites, which seem to have 

 been taken up mechanically by the liquid. 



The eastern and higher ridge is made up principally of the quartzites 

 and sandstones, which slope approximately with the surface of the ridge, and 

 curve somewhat with the spurs. They are well seen in section at the gap. 



