walcott.1 SUMMARY WYOMING. 349 



the advancing waters. The base of the Potsdam entirely around its outcrop is 

 a conglomerate. The ocean advanced upon the land with an action that has been 

 aptly termed that of a horizontal saw. The cliffs were undermined by its action, 

 the debris thus formed worn to bowlders, to gravel, and to sand. As the shore-line 

 advanced and a given point became farther from land, it received finer and finer sedi- 

 ments, until sedimentation, in this area at least, entirely ceased. 1 



He calls attention to the fact that the sandstone thins out towards 

 the central portion of the Black Hills uplift. Near Harney Peak the 

 last outliers are about 50 feet thick between the unconformable sub- 

 jacent "Archean" rocks and the undisturbed conformable superjacent 

 Carbouiferous limestone. 



WYOMING. 



There has been comparatively little detailed information published 

 relating to the Cambrian rocks of the Big Horn Mountains. Dr. E. V. 

 Hayden visited the range in 1861 and found the nucleus composed of 

 red feldspathic granite and a series of stratified Azoic rocks. Besting 

 on these occur the sandstones which are referred to the u Potsdam." 

 He found a few thin layers of fine calcareous sandstone filled with fos- 

 sils characteristic of the period. 2 These were studied by F. B. Meek, 

 who identified Conocoryphe and perhaps Dikelocephalus. 3 



According to Prof. T. B. Comstock, the sandstone referred to the 

 Potsdam attains a thickness of about 200 feet in the region of the Wind 

 Kiver Mountains, and above the sandstone on the eastern side, along 

 the central portion of the range is a series of limestones resting on the 

 sandstone, having a thickness of about 200 feet. From specimens col- 

 lected in the limestone he identified a trilobite of the genus Dikelo- 

 cephalus, several specimens of what was supposed to be Orthis tritonia, 

 and a quantity of a species of Theca. 4 As far as Prof. Comstock's ob- 

 servations extended the sandstone rested unconformably upon the sub- 

 jacent metamorphic rocks. 5 On the map accompanying the report the 

 "Potsdam" formation is represented as occurring all along the eastern 

 side of the Wind Biver range, and also along the central portion of the 

 western side. The latter occurrence, however, is denied by Dr. A. O. 

 Peale. 6 Dr. Hayden noted in 1862 the presence of a sandstone resting 

 upon the vertical edges of the Azoic clay slate series in the Laramie 

 Mountains, which he referred to the age of the Potsdam sandstone, 



1 Notes on the geology of the Black Hills. Preliminary Eep. Dakota School Mines, on the Black 

 Hills of Dakota, 1888, p. 31. 



2 Hayden, F. V.: Sketch of the geology of the country ahout the headwaters of the Missouri and 

 Yellowstone Rivers. Am. Jour. Sci., 2d ser. . vol. 31, 1861* p. 234. 



3 Meek, F. B. : Preliminary paleontological report with remarks on the ages of the rocks, etc. TJ. 

 S. Geol. Surv. of the Terr.. Sixth inn. Rep., 1873, p. 465. 



4 Geological Report. Report upon the reconnoissance of northwestern Wyoming, made in the sum. 

 mer of 1873, by William A. Jones. 1874, pp. 108-110. 



6 Op. cit.,p. 106. 



6 Report on the geology of the Green River district. U. S. Geol. Surv. of the Terr., lith. Ann. Rep,, 

 1879, p. 524. 



