BUCKSKIN AMPHITHEATER. 123 



the striking- large orthoclase crystals of the Lincoln Porphyry. The actual 

 line of contact of Blue Limestone and Weber Grits, occurring generally in 

 the covered gap of the depression between the ridge and the steeper mount- 

 ain slope, was seldom observed. It was therefore impossible to determine 

 whether the sheet of Lincoln Porphyry, which occurs above it on the higher 

 part of the mountain mass, extended eastward as far as the foot-hills or not. 

 Some outcrops of porphyry were observed which might have belonged to a 

 continuation of this sheet, but no facts of sufficiently definite significance 

 were obtained to justify its indication on the map. 



A good section of these outlying ridges is obtained in the narrow wind- 

 ing gorge of lower Buckskin Creek for about a mile above Alma. The 

 beds of the Weber Grits formation exposed along the walls of the valley, 

 which lie within the forest-covered belt, show much more decomposition and 

 disintegration than is found in the same beds above timber line. Thev con- 

 sist of coarse micaceous sandstones, with a considerable development of 

 argillaceous shales, also micaceous, and one or two thin beds of gray lime- 

 stone. Among the shales is conspicuous a black carbonaceous bed, and the 

 limestone is supposed to be that which occurs at about the middle of the 

 formation and which outcrops again in the wooded hills east of the Platte 

 Valley. The sandstone which immediately underlies the town of Alma 

 itself, and which is made up of grains of quartz about the size of duck- 

 shot, with considerable muscovite, might be mistaken at a little distance 

 for a decomposed granite. It shows but few bedding planes, and, though 

 in excavations for buildings it stands as a straight wall, when broken down 

 it crumbles at once into coarse sand. 



Buckskin amphitheater. This immense basin at the head of Buckskin gulch 

 bears the same relation to Mount Bross that the Platte amphitheater does 

 to Mount Lincoln, the two separating the Lincoln massive from the main 

 crest of the range, with which it is connected by the dividing ridge running 

 from Mount Cameron to Democrat Mountain. 



An excellent exposure of the Archean formation is afforded in its steep 

 walls, which rise 1,500 to 2,000 feet from the bottom of the basin and are 

 capped on the eastern side by a thin covering of Paleozoic beds. The 

 rocks are mainly gneisses and amphibolites, with local developments of 



