1888.] 501 [Crosby. 



ite, which appears to die out very gradually as we pass from the 

 centre of the Harney Range outward into higher and higher por- 

 tions of the great series of schists, was completed before the for- 

 mation of the eastern or newer Archaean series began ; while the 

 occasional occurrence of pebbles of feldspar and rose quartz in the 

 conglomerate, at the base of the Potsdam shows, as Newton has 

 pointed out, that the granite veins, like the enclosing slates and 

 schists, are unquestionably pre-Cambrian. That the Harney granite 

 is older than the newer series of schists is also quite clearly proved 

 by an observation made by my father, F. W. Crosby, in the north- 

 ern Hills. In the vicinity of the Monarch mine, east of Dead- 

 wood, the newer schists are overlain by the Potsdam beds, which 

 are traversed by large intrusive dikes and sheets of porphyry. At 

 some points the porphyry is so crowded with fragments of the rocks 

 which it intersects as to present the aspect of a breccia. It not only 

 encloses the Potsdam quartzite and shale, but fragments of the un- 

 derlying slates and schists are especially abundant, and an occa- 

 sional fragment of granite precisely similar to that of the Harney 

 Range is observed. These have certainly been brought up by the 

 lava from some formation or horizon below the newer Archaean 

 series, which, with the exception of the Potsdam, forms the surface 

 for miles in every direction from this point. 



Paleozoic and Mesozoic Formations. 

 The two most salient facts in the stratigraphy of the Black Hills 

 are the perfect unconformity observable everywhere between the 

 Potsdam sandstone and the underlying Archaean schists and slates ; 

 and the equally perfect conformity between the Potsdam and the 

 overlying formations. The entire conformity of the Potsdam and 

 Carboniferous is a universal fact, which is clearly exhibited in nearly 

 every canon and escarpment throughout the length and breadth of 

 the Hills. 



Conformable Contact of the Cambrian ayxd Carboniferous. 

 The conformity of formations which represent essentially con- 

 tinuous deposition, like the Paleozoic series of the Appalachian 

 region, should not excite surprise, even though the oldest and 

 newest strata in the series — the Potsdam and Carboniferous — are 

 separated by an enormous interval of geological time. But the 

 case is different in the Black Hills, for the conformable Potsdam 



