‘? 
465 
Mr. Newton gives a synopsis of the strata of the Black Hills, of which 
the following is a short abstract: 
Ages. | Feet. Strata. 
| 
Cenozoic: 
Miocene ....-.-....--22----- 22-22 e ee 200 | Clays and conglomerates. 
Cretaceous..------------------------- 900 | Clays and shales; sandstone. 
Mesozoic : | 
Jura... -- eee eee eee ee eee ee ee eee 200 | Clays and marls, with some limestone. 
Red Beds ...--..---------2---2 ------ 340 Red clay. 
Carboniferous ....--....--.---------- 690 | Sandstones and limestones. 
Paleozoic: 
Silurian (Potsdam) .....----.-.------ 250 | Silicious sandstones and conglomerates. 
Archwan (?)..-.--------2- 0-22-2222 eee ee eee eee Slates and schists, with intrusive granite. 
Of these the Mesozoic and Paleozoic strata are resting conformably 
upon each other, the Cenozoic and Archean are not. The Black Hills 
have been formed by the uprising of the Archean rocks, which lifted 
up and broke through the overlying strata. That this uprising must 
have taken place after the Cretaceous and before the Miocene forma- 
tions can be seen from the fact that the strata of the latter do not con- 
form with those of the former. 
The Black Hills have received their present form by erosion. The 
softer rocks have worn away faster, leaving the harder standing out as 
ridges or crags. As the center is raised, the dip of the strata is out- 
ward and the outcroppings form concentric ovals. The Jurassic clays 
and marls, the clays of the Red Beds and the slates and schists of the 
Archean formation are comparatively soft, while the Cretaceous sand- 
stone and the granite spurs of Archiean formation are hard. The 
Miocene formation belongs to the surrounding plains and does not 
enter the Hills anywhere. 
The outcrop of the Cretaceous sandstone forms the foothills, and that 
of the Jura formation and the Red Beds makes the so-called “ Race 
Track,” a more or less continuous valley between the foothills and the 
hills proper. The Race Track is much broader on the south side and 
forms the larger part of the ‘ Minnekahta Plains.” On the east side 
the Carboniferous limestone belt forms a series of hills, and on the west — 
side, where the strata are nearly horizontal, a broad plateau, the ‘“ Lime- 
stone District.” As the slates and schist are comparatively soft, the 
center of the Black Hills is lower, except where the granite or igneous 
rocks come to the surface and form the highest peaks in the range. 
ALTITUDES. 
The plains at the base of the Black Hills have an altitude of about 
1,000 meters. The highest point is the top of Harneys Peak, one of 
the series of granite crags at about the center of the Hills, Its alti- 
tude is given differently. The most reliable measurements are without 
doubt those given by the United States Geological Survey, one of 
which makes it 7,368 feet, the other 7,440 feet, or, respectively, 2,245 
