PEAtE.] DESCEIPTIYE GEOLOGY GEEEN EIVEE BASIN. 539 



Their age must remain doubtful until a section in the canon of the 

 Piney is made. The sandstones forming the upper portion of the section 

 are undoubtedly the Laramie sandstones ; for, on a branch of the Piney, 

 only a little farther north, I obtained Laramie fossils. The relation of 

 the fold to the plateau could not be definitely determined. The simplest 

 solution appears to be that the plateau represents the highest portion 

 of the elevation, and that in the fold north the overlying beds. have not 

 been removed, but that Jurassic and Cretaceous layers are still pre- 

 served. This supposes a vast erosion subsequent to the uplift; for, on 

 the east of Thompson Plateau, the Wahsatch beds rest on the Carbonif- 

 erous limestones extending to the eastward in long, smooth slopes, while 

 at the canon they rest on the more modern beds, whether Jurassic or 

 Cretaceous I am unable to say. It is probable that Cretaceous is pres- 

 ent, as there is room for a thickness of 7,000 feet of beds between the 

 axis of the fold and the range to the westward. 



MERIDIONAL VALLEY. 



Between the rim of the Green Biver Basin and the Wyoming Bange 

 is a deep, trough-like depression, which, as far as could be determined 

 by the few fossils that were found, is filled with sandstones of the Lar- 

 amie Group. The structure of this valley will be apparent by glancing 

 at the sections across it represented in the accompanying plate, showing 

 sections of Meridian fold. The valley extends from the Piney, north of 

 Thompson Plateau, to Marsh Creek Valley. The branches of the Piney, 

 Bitterroot, and Marsh Creek drain the region with north and south 

 courses, while the main streams flow eastward. The rim of the basin is 

 an anticlinal, which is represented in the sections. In this anticlinal no 

 rocks older than the Jurassic were observed. On the north side of the 

 main Piney the steepest side of the anticlinal is the eastern, the dip of 

 the beds being 80° on that side as seen beneath the unconformable Wah- 

 satch beds. The western side dips west 50°. The lower beds are prob- 

 al )ly of Jurassic age, as determined by fossils found farther north. They 

 were seen from a distance at this point, and consist of gray beds, probably 

 bmestones. Above them a reddish limestone outcrops, followed by sand- 

 stones and shales, and again white sandstones with greenish-gray lam- 

 inated sandstones still higher. There is room in the section for 7,000 

 feet of beds, and the whole Cretaceous system is probably present, 

 although not positively identified. The dip of the beds on the west side 

 of the anticlinal is south 77° west. In the range to the west the dip is 

 westward. On the east the variegated Wahsatch beds rise on the ridge 

 so as to cover the greater part of the eastern side of the anticlinal. They 

 dip 6° to the eastward. 



On a small branch of the Lake Fork of Piney Creek, east of Station 

 48, 1 found a Corbula, and other indistinct fossils. On the north side of 

 the main Lake Creek the following were obtained : Pentacrinus asteriscus 



and Neritina f The angle of dip here on the western side of the 



anticlinal is about 25°, as shown in the section. There appear to be 

 quartzites in the centre of the fold, but whether they are the representa- 

 tives of the Bed Beds or not, it is impossible to say. The fold at this 

 point is much more gentle than on the main Piney. The section on the 

 south side of Bitterroot Creek shows the fold to be still more gentle, the 

 western members showing dips of 20°. The eastern side is still the more 

 abrupt, and the Wahsatch beds still conceal the greater portion of it. 

 Xorth of the Bitterroot the Wahsatch beds cover the axes of the fold 

 entirely, and present a bluff face to the westward. On top the beds are 



