208 < . r. v.vx ftisf: — pre-cambrian of the black hills. 



to the southward, slates are again found. Rochford is about one-third the 

 way south in the pre-Cambrian core and near the line dividing the two sup- 

 posed series. From this place excursions were made both east and west, the 

 first of which ought to traverse the slate area and the second the schist area. 

 So far a> could be made out, the rocks were not more crystalline west than 

 east. Passing southward from Rochford toward Hill City, the course of 

 travel was such that it crossed and recrossed Newton's boundary between 

 the slate and schist series. While to the south the rocks were found to 

 become more crystalline, no difference in this respect was observed east and 

 west. The slate area was mapped as coming directly in contact with the 

 granite in the southeastern part of the pre-Cambrian area. North of the 

 granites were found, for some distance, as thoroughly crystalline schists as 

 anywhere iu the Hills, the rocks becoming less crystalline, however, toward 

 the north. .V journey was made around the granitic area, and all the way 

 crystalline schists were found surrounding it. These schists everywhere 

 strike parallel to and dip at a high angle away from the granitic core. To 

 a certain extent these relations were noted by Newton, and his observations 

 are verified by Crosby, although neither reached the above generalization. 

 They are of such interest that Newton's words are quoted.* He says : 



•■ West el' Harney the strike "f the rocks is from north and south to northwest and 

 southeast, and we find the inclosed granite masses running in the same manner. 

 Southward, on French creek at and above tie' stockade, the strike of the schists is 

 changed, and with them the inclosed granite ridges run nearly east ami west. South- 

 west of the stockade, in Custer park, the schists and granite run north and south, 



and this strike is exchanged in tl astern pari of the park region for an east and 



!, which bends around on the easl side of Harney, becoming the customary trend' 



toward the north and north west." 



****** 

 " Tin: dip of the schists is usually very high and often vertical, though occasionally 



by local variation it becomes quite low. In several places a differen< f dip was 



noticed between the schistose rock- on the west and the Blates on the northeast side 

 of Harney peak, the former being toward the wesl and the latter toward the east, 

 but the number of observed point- oi variation was not sufficient to warrant the 

 temenl that this difference is a p M feature of the relation of the two series 



<,f rock-. Tin-re i- found a change corresponding to the change in strike 



already noticed on French creek, and the dip becomes slightly southward from the 

 vertical. On the headwaters of Red Canon creek it is 70° to v o south; on lower 

 French cr< outh." 



Neither New ton nor < Irosby -ay anything about the relative strike of tin' 

 Blaty and Bchistose rocks north of the granite, although adjacent to it is 

 drawn the line dividing the two supposed series. < >ur examination showed 

 here, as elsewhere, that the schists strike parallel to the granite -i. e., in an 



of Pa 



