I 00 GEOLOGY OF THE BLACK HILLS. 



iferous outcrop of the Potsdam, and to expose imperfectly the following 



strata : 



(3) Shaly limestone, 15 feet in thickness, and full of fragments of 



crinoidal columns. 



(2) Talus of 135 feet with imperfect outcrops of strata of red sandy 

 conglomerate, and at base a reddish conglomerate with calcareous cement. 



(1) An outcrop, 10 to 15 feet in thickness, of reddish and light colored 

 sandstone, somewhat conglomeratic at base, containing Lingulepis, Oholella, 

 and trilobite fragments. 



From the summit of this butte the edge of the limestone plateau to 

 the south and west can be seen for a long distance running out eastward 

 in peninsulas and cut back westward by valleys, with occasional isolated 

 buttes standing like sentinels along its serrated margin, while eastward is 

 the rugged area of the slates, with now and then a grassy park, and in the 

 distance the dark range of Harney. 



Southward from these points of observation the Potsdam was not 

 examined until it w^as reached between the headwaters of French and Red 

 Canon Creeks. The exposures are poor, being generally mere fragmentary 

 outcrops on the slopes below the limestone cliffs. The cliff tops at the head 

 of Red Canon Creek are GOO feet lower than those on Castle Creek, show- 

 ing a marked slope of the table toward the southwest. Levels taken at two 

 places between approximate limits of the Potsdam gave a thickness at each 

 of 220 feet. At the springs near the head of Red Canon Creek a purplish 

 quartzite was found at the base of the Potsdam resting on the schists, and 

 from its abundant fossil remains an excellent collection of Lingidepis and 

 Oholella was obtained. 



Eastward from this point the formation was next examined in the broad 

 prairie that lies on upper Amphibious Creek. Here the hills, capped with 

 a cliff of 20 feet of limestone and a fragmentary outcrop of the underlying 

 lower portions of the Carboniferous, contain along their base imperfect 

 exposures of a brownish sandstone of the Potsdam with Lingulepis, etc. 

 Some five or si.K miles down the creek, at the head of the Great Canon, are 

 heavy beds of the gray basal (piartzite, and the slope above it is strewn 



