66 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



of tlie two takes place, and the former continues in a sontlieasterly course. 

 It follows a line slightly diverging from the longitudinal axis of the 

 main range. We find that the continuity has here also beeu broken, 

 and streams or creeks find their way through the gaps that have been 

 formed. 



GEOLOGICAL CHARACTEK OF THE RANGE. 



Prozoic roclis. 



Beginning with the subsidiary range along the western base of the 

 mountains, we find that it is composed of a comparatively coarse-grained, 

 red granite. Structureless, the hills rise to a relative elevation of about 

 1 ,000 feet. From a distance they appear as if composed of a large number 

 of bowlders piled on top of one another. Weathering in rounded forms, 

 the hUls, in spite of their small size, appear to be quite massive. The 

 granite is comiiosed of orthoclase, quartz, and muscovite. Of these the 

 former has a flesh color, the second is gray. Admixtures of minute par- 

 ticles or crystals of magnetite impart to the rock a bright-red color upon 

 exxiosiu-e. This is the result of a higher degree of oxidation of the 

 magnetite. After the hills are fairly separated from the western slope 

 of the main range they are bordered on either side by young sedimentary 

 strata. As these show a nearly horizontal arrangement, the granitic 

 hills appear like islands above them. Extending eastward the granites 

 disapijear, and schists take their place. So far as could be determined, 

 the latter merely form a covering, hiding the former from sight. 



This occurrence of prozoic rocks is one of great interest. Although 

 in close proximity to the Wind EiverEange, this latter shows no granite 

 of the same character. In our whole district it appears but twice more, 

 and each time it exhibits the same features here observed. The gran- 

 itic outcrops near the Sweetwater Eiver, south of Saint Mary's Station, 

 and again a portion of those formed by the Granite Hills show the same 

 rock. From this we may argue an existing subterranean connection, 

 one which during the earliest i)eriods of sedimentation was exposed and 

 unbroken. I regard this granite as the oldest rock in the Wind Eiver 

 Mountains, a view which is sustained by an absence of all structure, its 

 relative position to the range, and its relations to the undoubted meta- 

 morphic rocks farther east. 



MetamorpMc roclcs. 



Upon examination we found that the prozoic granite above described dis- 

 appeared altogether in the main chain, except northward, where it formed 

 part of its western slope. The hi ghest portions of the Wind Eiver Mount- 

 auis are composed entirely of metamorphic rocks. Granites mainly, with 

 some schists, build up the main chain. In color, composition, and mode 

 of weathering the granite here diifers from that immediately west. It 

 is hard, fine-grained, of white to gray color, and composed of orthoclase, 

 oligoclase, quartz, muscovite, and some phlogopite. Within certain 

 zones, which are not entirely constant however, amphibolite replaces the 

 micas, either partly or entirely. Syenitic granite or Fyenite is the rock 

 produced by this change. Accumulations of either mica or quartz at 

 some locahties change the character of the rocks. We have, altogether, 

 a typical occurrence of metamori^hics, one exhibiting many of the varia- 

 tions to which this class of rocks is subject. 



Although special examinations were made to determine whether the 

 different mineralogical constitution of the rocks remained constant within 



