50 GEOLOGY OF THE BLACK HILLS. 



formed of the Potsdam sandstone and the Carboniferous limestones. This 

 escarpment faces inward or toward the Archaean area, and in the canons 

 which cut into it the slates and schists are seen beneath the sandstone of 

 the Potsdam which rests unconformably upon them. With the exception 

 of Harney and a few of the neighboring peaks, the average elevation of 

 the schist and slate area is considerably below that of the encompassing 

 sedimentary rocks. Its relative depression is plainly due to the more rapid 

 decay and degradation of the schists and slates, while the hard limestones, 

 being less destructible, remain prominent. 



Besides this main area of the metamorphic rocks, there are a number 

 of minor exposures to the north and northwest of Custer and Terry 

 Peaks. The countiy is there underlaid by the Carboniferous limestone, 

 and this, together with the underlying Potsdam, has been cut through in 

 deep and narrow canons with innumerable branches, making the country a 

 perfect labyrinth of canons many of which are 400 to 600 feet in depth. 

 The main canons have been eroded also many feet deep into the underly- 

 ing Archaean, and in some places the gold concentrated from its slates and 

 schists has been collected in very considerable quantities. 



The metamorphic rocks of the Black Hills are separable into two dis- 

 tinct groups, whose lithological characters are marked and persistent. 

 Their stratigraphy was carefully studied in the hope that it would be pos- 

 sible to definitely determine the historical relation between them, but the 

 result was not satisfactory. The great differences, however, in the charac- 

 ters of the rocks are sufficient to warrant their separation into a western 

 series or group of schists and an eastern series or group of slates. The line 

 of separation between them can be only imperfectly indicated. Its trend, 

 so far as could be ascertained, is a little west of north. Starting just east 

 of the granitic range of Harney Peak, it curves westward about the north 

 end of that range, and then turns toward the north-northwest, passing near 

 the forks of Spring Creek or the present site of Hill City, crossing Castle 

 Creek in the west canon, and disappearing beneath the Paleozoic rocks in 

 the vicinity of Custer Peak. To the north of the peak it reappears with a 

 northerly course. 



