474 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



CHAPTER V. 



AECILEAN AEEAS. 



In the extreme southwest comer of the district surveyed, a limited 

 belt of Archsean rocks extends northward from the Mount Putnam zone, 

 but is much obscured, if indeed it shows a fair exposure at all in our 

 district, by Pliocene and detrital materials which cover the western 

 flank of the mountain immediately south of the canon of Eoss Pork. 

 But just over the line within the territory assigned to Messrs. Peale and 

 Gannett, these rocks attain the magnitude of heavy deposits, consisting 

 of green chloritic slates and quartzites, which in Mount Putnam are 

 tilted into a vertical position, with perhaps slight general inclination 

 from the vertical a little south of east. Still farther to the south they may 

 be advantageously studied in the section along Portneuf Eiver where it 

 cuts across this mountain ridge on its way to the Snake plain. 



Over the mountainous region extending east from the Snake plain, in 

 which is found a system of extraordinary plications in the earth's crust, 

 none of the ancient crystalline rocks are brought to view, unless such 

 may have appeared in some of the deep basin depressions, where, how- 

 ever, subsequent effusions of volcanic matter and accumulations of 

 quaternary materials have sealed from view the vestige of these rocks. 

 It is not until we approach the culminating crest of the district that 

 these rocks are encountered, and where their erosion has produced some 

 of the most sublime and beautiful mountain scenery in the Northwest. 

 The nucleal rocks of the Teton Eange are composed of rocks of this age. 

 Their distribution and general characteristics have already been no- 

 ticed in the detail account of the examination prosecuted in this 

 quarter, and it is only necessary to briefly mention their occurrence in 

 this place. 



It is evident, even after so cursory study as the past season allowed, 

 that the metamorphic or gneissic and schistose varieties of rocks con- 

 stitute by far the most prevalent rocks of the Archoean series in this 

 region. But in the cluster of peaks surrounding Mount Hayden as well 

 as the noble peak itself, a light and flesh-colored granite forms the core 

 upon which atmospheric erosion has wrought with so wonderful results 

 in mountain architecture. As seen from the opposite side of Jackson's 

 Basin, the mass in the great peak seems to have a bedded structure, with 

 dips, according to Professor Bradley, but little from the vertical ; but 

 from the west the mountain presents vertical precipices of enormous 

 height, in which the granite appears in huge angular blocks or slab-like 

 masses with j>ohshed, glistening surfaces. To the north the Archaean 

 area expands, occupying quite the entire breadth of the range along a 

 line drawn through Mount Moran, and over the greater part, if not the 

 whole, of this bared area the rocks are gneissic, or such their peculiar 

 sharp, jagged manner of weathering pronounces them. Immense sur- 

 faces of the rocks in this quarter are laid bare by erosion, in which 

 former glaciers may well be credited with an ample share. The crest of 

 this expanded belt forms a iprominent spur which abruptly terminates 



