486 DESCRIPTIVE GEOLOGY. 



cretions of tiidymite, with a little quartz, some biotite, apatite, and horn- 

 blende. Adjoining this is a rhyolite showing small white opaque feldspars, in 

 a purple, somewhat rough-feeling groundmass, carrying small, green,^brous 

 hornblendes, but no macroscopical quartz. Under the microscope, no tridy- 

 mite is found, but the sanidins are seen to contain glass-inclusions, having 

 a peculiar serrated border not found in the inclusions of quartz. 



In the Scliell Creek Mountains, which extend to the south from Leech 

 Springs, forming the eastern border of Steptoe Valley, are limestones in 

 which Primordial fossils have been found, overlying heavy bodies of Cam- 

 brian quartzite; the northern portion of the range is, however, entirely 

 covered by flows of rhyolite, as far as has been observed. 



Among the organic remains from this range, two new species have been 



identified : 



Crej)icejpJialus {Loganellus) anytus. 



Lingulepis Mcera. 



Steptoe Valley has at its lowest point, where the waters which di-ain from 

 the Egan Mountains form a little lake, an elevation of 5,730 feet above sea- 

 level, already 600 feet more than that of Deep Creek. 



Egan Range. — Tliat portion of the Egan Range which is represented on 

 this map is a high, narrow limestone ridge, about 6 miles in width, having 

 a north and south trend, whose most lofty peak rises nearly 5,000 feet above 

 the sink of Steptoe Valley, while its average width is only about 6 miles 

 from base to base. In topographical structure, it presents a double crest, a 

 central line of depression being formed by a series of high, interior, longi- 

 tudinal valleys, which di-ain to the eastward. Its general geological struct- 

 ure is that of an anticlinal fold, along whose crest erosive action has been 

 the greatest, resulting in the formation of these longitudinal valleys, ^vhose 

 direction has doubtless been determined by a line of faulting along the axis 

 of the fold. As compared with most of the desert ranges, it is unusually 

 well watered, there being numerous large springs issuing from tlie lime- 

 stones along its base, and considerable streams running from the interior 

 valleys ; that from Gosi-Ute Canon empties into the little lake at the sink of 

 Steptoe Valley, and that from Pleasant Valley sends a stream of water for 

 many miles out on to the desert, while its higher slopes and the interior 



