430 DESCRIPTIVE GEOLOGY. 



ular intervals, west of the Terrace Mountains, are small isolated buttes and 

 knolls of Tertiar}^ volcanic rock.' In exceptional instances, as at Desert 

 Butte, they rise from 500 to 600 feet above the plain, but in general are 

 small, low hills, mostly concealed by the Quaternary deposits. Several of 

 the smaller outcrops occur along the south and southeast spurs of the Eaft 

 River Mountains, but occupy so small an area that they have not been des- 

 ignated on the maps. So far as examined, they appear all to be rhyolites, 

 with considerable variety in texture and aspect, but in general more allied 

 to the compact, porcelain-like kinds than to the rough, porous types. The 

 rhyolite of Desert Butte, which is situated near the wagon-road, some 5 

 miles to westward of the Raft River Mountains, may be considered as a 

 typical variety. In its physical habit, it is a dense, compact rock, exhibit- 

 ing to the naked eye almost a homogeneous groundmass, and breaking 

 under the hammer with a sharp angular fracture. In color, the prevailing 

 tint is a light reddish-gray, shading off into white or salmon. The rock 

 would be very uniform in texture but for the occurrence of rough sphseru- 

 litic bodies and the characteristic lithophysse. Narrow seams and lines 

 of semi-transparent chalcedony traverse the rock in various directions. In 

 a hand-specimen, the rhyolite somewhat resembles a quartz-porphyry, with 

 small grains of brilliant white quartz and feldspar scattered through the 

 groundmass; except that the minute feldspars have the characteristic lustre 

 and habit of volcanic rocks. Under the microscope, the quartz-grains are 

 seen to abound in glass-inclusions. Professor Zirkel has detected the presence 

 of tridymite, occurring in hollows and cavities of the rock, and suggests that 

 it is a secondary product, as it is often seen overlying iron-ochre and earthy 

 ferrite, which are themselves of later origin than the rhyolite. Well-marked 

 terraces form a prominent and somewhat curious feature of the Desert Buttes. 

 The broad terrace measures 6 to 8 feet in width, being nearly level, and 

 about 125 feet below the summit. This same terrace may be traced with 

 a pocket-level for a considerable distance along the shore of the northern 

 bay of the old lake, extending up the valley, west of the Raft River Mount- 

 ains. Four distinct, plainly-marked terraces may be seen from this point, 

 showing at favorable points level benches, 30 to 40 feet in width, cut in the 

 solid rock, below which the slopes shelve off very abruptly. 



