MOPUNG HILLS. 753 



south for about 4 miles, and directly beyond it occurs an outburst of basalt, 

 which seems to break up between the Triassic slates and an unconformable 

 overling series of Tertiary sandstones, dipping to the westward at an angle 

 of about 40°. These sandstones are composed of grains and pebbles of 

 quartz, associated with some calcareous and argillaceous material, closely 

 resembling those of the Kawsoh Mountains, and, from their inclined posi- 

 tion, are referred to the Truckee Miocene rather than to the Pliocene series. 

 They contain no fossils. The basalt has poured over them after considera- 

 ble erosion, occupying the basin-like depression on the edges of the 

 upturned beds, and following down the lines of Tertiary drainage. This 

 furnishes another argument .for referring them to the same age as the Mio- 

 cene of Fossil Hill, Kawsoh Mountains. Along the western skirts of these 

 basaltic hills, the characteristic calcareous tufas of the Lake La Hontan Basin 

 are well shown. About a mile out from the foot-hills basalts, and rising out 

 of the Quaternary, is another table of basalt, not unlikely a part of the same 

 flow, which is altogether covered with the same tufa. South of the meta- 

 morphic uplift the country is occupied for about 10 miles by low, irregular, 

 rhyolitic hills lying along the trend of the range. This group, although it 

 has been but little studied, appears to resemble in its mode of occurrence 

 and petrographical habit the Mopung Hills to the south, the only specimens 

 of the rock obtained showing a similar groundmass of brick-red, white, and 

 cream-colored bodies. 



South of here for 8 miles, the range is again occupied by sedimentary 

 rocks inclined to the south and southwest. They consist of highly meta- 

 morphosed quartzites and quartzitic slates, over which is a bed of highly 

 altered limestone. Neither in the quartzites nor limestones were any fossils 

 discovered, and, since the body is entirely isolated from other sedimentary 

 regions, there is no sure method of arriving at its age. The quartzitic 

 schists, however, so closely resemble those which form the uppermost mem- 

 bers of the Koipato series, that they have been referred to the same 

 horizon, overlaid by the lowest stratum of the Star Peak limestone, although 

 it is by no means impossible that hereafter some organic remains may 

 be obtained referring them to some higher geological position. 



Mopung Hills. — The Mopung Hills terminate the West Humboldt 



