IBENPAH MOUNTAINS. 473 



are very long and gradual, occupying a width of 4 or 5 miles in a descent of 

 less than 1,000 feet. On these slopes, the ancient lake-terraces can be 

 traced, from a little distance, with great distinctness, but, owing to the low 

 angle of the slopes, their actual level is difficult to determine at any one 

 point. The highest terrace which could be distinctly recognized stands at 

 an elevation of 800 feet above the desert-level, and is composed entirely of 

 loose gravel material. The pebbles consist principally of limestone, with 

 some mixture of quartzite and granite. In this upper terra,ce was found no 

 tufa. At the level of 500 feet above the Desert is a very well-defined ter- 

 race of calcareous tufa, filled with casts of recent fresh- water shells of the 

 species common in the Lower Quaternary deposits of the Desert. Of this 

 tufa, an analysis was made by Mr. R. W. Woodward, with the following 

 result: 



Silica 8.40 8.22 



Lime 46.38 46.50 



Magnesia 3.54 3.52 



Alumina 1.31 1.20 



Soda.. 0.48 . 0.54 



Potassa 0.22 0.21 



Carbonic acid . 38.20 38.33 



Phosphoric acid trace trace 



Water 1.71 1.62 



100.24 100.14 

 (Silica combined, and as free sand.) 



A second prominent gravel terrace was found to be only 300 feet above 

 the desert-level. Some ten or twelve distinct lines of intermediate terraces 

 could be observed along the slopes of the range. These heights were all 

 determined by barometrical measurements referred to station-observations 

 at Redding Springs, a group of large pools of tolerably pure water, occupy- 

 ing the edge of the Desert, just east of the stage-road at the base of the 

 extreme eastern slopes of the Ibenpah Range. 



The valley of Deep Creek is traversed by a stream of this name, which 

 takes its rise in the high peaks of the Ibenpah Mountains, to the south of 



