176 DESOEIPTIVE GEOLOGY. 



mica. In this respect, as in its general appearance, it resembles some of 

 tlie lavas of the Leucite Hills in the Green River Basin. Besides the mica, 

 no crystals can be distinctly recognized by the unaided eye ; but the micro- 

 scope discloses the presence, in the groundmass, of crystals of sanidin, 

 with a few plagioclases, frequent augites, and a few olivines, but neither 

 quartz nor hornblende. The characteristic mineral, however, is one which 

 shows colorless rectangular sections, with fibrous borders, which resembles 

 nepheline, though it wants its usually sharply-defined hexagons. As the rock, 

 when powdered and treated with hydrochloric acid, gives a precipitate' of 

 gelatinous silica, there can be little doubt of the presence of nepheline, 

 more especially as the basalts of the surrounding hills are all nepheline- 

 basalts. A specimen of the rock from the head of Slater's Fork was sub- 

 jected to chemical analysis by Mr. R. W. Woodward It has a high specific 

 gravity, 2.7, and contains the following ingredients: 



Silica - - 53.12 53.25 



Alumina ' .-. 14.54 ]4.42 



Ferrous oxide 6 01 6.00 



Manganous oxide trace trace 



Lime 6.01 6.01 



Magnesia 5.20 5.06 



Soda 3.02 3.13 



Potassa 4.54 4.58 ■ 



Water 7.58 7.63 



100.02 100.08 

 Basaltic Hills. — The basalts of the Elkhead Mountains belong, with 

 few exceptions, to the group of nepheline-basalts, containing little or no feld- 

 spar, but in general considerable olivine, with augite and magnetite. They 

 form the main east and west ridge west of Whitehead Peak, and numerous 

 isolated, picturesque hills north of this ridge. The valley of Slater's 

 Fork, which has been eroded out of the basaltic hills, discloses the sand- 

 stones, which underlie the flows, in a few isolated outcrops, but the forests 

 are too thick to allow the tracing of the line of contact between these and 

 the basalts with any great degree of accuracy. 



In one exposure on the north face of Anita Peak, however, the white 



