LARAMIE HILLS. 7 



above the fresh- water Pliocene strata of the Great Plains. Again, the beds 

 are observed in places as low hills, with gently-inclined strata; at others, 

 they are carried up in abrupt, bold ridges, the inclination of the beds vary- 

 ing from a nearly horizontal position to abrupt walls standing quite vertical. 



As seen just south of Wahlbach Spring, on the Cheyenne Pass road, 

 they appear nearly horizontal, having been elevated nearly 1,000 feet above 

 the plains, while immediately north of the main stream of the Chugwater 

 they stand out, in curiously-moulded ridges with beautifully-curved outlines, 

 at 88°, and at one locality pas* the vertical position. What may be said 

 in detailed description of the structural relations of the sandstone and lime- 

 stone beds is deferred till after the discussion of the Archsean bodies,, and 

 are only mentioned here to give a general outline of their bearing upon the 

 great mass of highly crystalline rocks that really make up the range. By 

 reference to the upper section at the bottom of Geological Map I, eastern 

 half, the structure may be seen. The section crosses the Laramie tlills in 

 an east and west line, passes just north of Wahlbach Spring through San- 

 ders Peak, and cuts the plain near Laramie City. 



Aech^an Geolosy. — The Archsean rocks of the Laramie Plills, while 

 presenting a considerable variety in texture and physical appearance, may 

 nearly all be classed under granites, gneiss, mica- and hornblende-schists ; 

 and, of these, granites cover much the largest area. They probably belong 

 to the very oldest rocks observed on the survey of the 40th parallel, and 

 may be placed with but little hesitation in the Laurentian series ; their 

 petrographical structure, combined with certain lithological and mineralog- 

 ical characters, strongly indicate, if they do not clearly establish, an equiv- 

 alent age with the metamorphic granitoid rocks developed in Canada, New 

 York, and Is^orthern Europe. 



This Archsean series consists of a central body of coarse-grained granite, 

 made up of quartz, feldspar, and mica, but apparently with little structure. 

 Above the latter, and forming the outer edges, dipping east and west away 

 from the main mass, occurs a hard, compact, heavily-bedded series of reddish 

 granitoid rocks composed of quartz and feldspar. At the north and south ends 

 of the range, the granites graduall}'' pass into a laminated structure, and the 

 rock-masses become well-defined gneisses and schists. As already indicated, 



