PEOMOl^TOEY MOUNTAINS. 421 



arms of Salt Lake, with a varying width from 4 to 7 miles, and reaching in its 

 highest point 3,000 feet above the level of the lake. North of the railroad the 

 range is comparatively low, with rounded outlines, the greater part of its sur- 

 face being covered with loose soil and grass, and showing but few outcrops. 

 The underlying formation, however, belongs to the Wahsatch limestone, and 

 is evidently a continuation to the northward of the same beds which charac- 

 terize the more important portions of the range projecting into the lake. 

 The railroad passes through a low depression in the range, which, on the 

 summit, attains an altitude 4,943 feet above sea-level, or over 700 feet above 

 the level of Salt Lake. The old Pliocene lake, at its highest elevation, 

 unquestionably occupied this pass, isolating the main portion of the Prom- 

 ontory Mountains, which formed an island of greater extent than either 

 Stansbury or Antelope Island. This gap in the range, at its widest 

 expanse, measures about three miles in a north and south direction, and 

 everywhere shows the rounded forms and broad level benches produced by 

 recent erosion and former occupation by the lake waters. On both the 

 north and south sides of the gap, the upper terrace-lines of the old lake 

 are quite marked, but perhaps less continuous than at other localities. All 

 along the east and west sides of the Promontory Mountains, these old ter- 

 races and beach-lines may be traced with more or less distinctness, indicated 

 by loose deposits of sand and gravel, or by benches cut in the hard mass 

 of limestone. The elevation of the highest of these terraces is approxi- 

 mately 940 feet above the present level of the lake. To the south and 

 west of the railroad, at Promontory Station, the range, which is quite nar- 

 row, consists of a series of limestones, of a prevailing gray color, in the 

 lower part of which are dark heavy beds of nearly black limestone, all dip- 

 ping to the westward at an angle of 38°. About four miles south of Prom- 

 ontory Station, the range widens rapidly to the westward, attaining a width 

 of 6 to 7 miles, of which the western third is occupied by the same series 

 of limestones, which here rise with an easterly dip, and overlie a limited 

 outcrop of Archaean schists. 



These Archaean rocks are exposed on the southwest corner of this pro- 

 jection of the range, and consist of quartzites and mica-bearing schists, 

 closely resembling those described in the Archaean bodies of the Wahsatch. 

 The main crest to the east of this western projection is occupied by con- 



