walcott.] SUMMARY — SOUTH DAKOTA. 347 



The presence of the Potsdam terrane on the south side of the Adi- 

 rondacks, along the valley of the Mohawk, has not been positively 

 proved. At Little Falls a thin layer of sandstone, resting unconform- 

 ably upon thepre-Paleozoic rocks and beneath the Oalciferous sandrock, 

 has been referred to the Potsdam. The discovery by Profs. Shaler and 

 Williams of a thin layer of shale, resting upon this layer of sandstone, 

 on the north side of the river at Little Falls, in which specimens of 

 Lingalepis acuminata occur, lends more authority for the statement that 

 the Potsdam is present; but as the species Lingulepis acuminataT&nges 

 up into the Calciferous sandrock, both at Whitehall, New York, and 

 on the north side of the Adirondack Mountains in St. Lawrence County, 

 it is doubtful if we can claim the presence of the Potsdam at any point 

 in the Mohawk Valley. There is not, however, any definite reason why 

 a thin bed of it, with the typical Potsdam fauna, should not be found 

 beneath the Calciferous, if it is not concealed by the overlap of the 

 superjacent Calciferous and Trenton terranes. 



The sediments of the Adirondack sub-Province and its Canadian ex- 

 tension, with a single exception, were accumulated in a shallow sea not 

 far removed from the shore-line. Eipple-marked sandstones and trails 

 of marine invertebrates occur. The sandstone is relatively tine-grained 

 with the exception of a small deposit of conglomerate in St. Lawrence 

 County, New York. The formation belongs to the Upper Cambrian, 

 and overlaps upon the sloping shores of the pre-Cambrian land. The 

 one exception to the arenaceous deposit are the limestones in Saratoga 

 County, New York, which were deposited on sandstone and overlap on 

 to the pre Cambrian rocks. The fauna of the limestone is the same as 

 that of the quartzite at Marble Eiver, near Chateaugay Chasm, on the 

 northern side of the Adirondacks. 



WESTERN BORDER OR ROCKY MOUNTAIN SUB-PROVINCE. 



This includes the Cambrian rocks of South Dakota, southern central 

 Montana, central and northwestern Wyoming, and Colorado. 



SOUTH DAKOTA. 



The chief summary of our knowledge of the Cambrian rocks of the 

 Black Hills of Dakota is taken from the reports of Messrs. Newton and 

 Carpenter. The sandstone identified as the u Potsdam w is the lowest 

 member of the fossiliferous series, and in numerous and excellent ex- 

 posures its character and relations are easily determined and studied. 

 It everywhere rests unconformably upon the upturned edges of the un- 

 derlying Algonkian slates and schists, filling up irregularities in their 

 surfaces, and its basal member is generally formed of coarse materials 

 derived from the erosion of the subjacent rocks. Consisting mainly of 

 coarse and friable sandstones, with conglomerates easily eroded, it 

 covers superficially very limited areas. 



By a reference to the geological map accompanying the report of Mr. 

 Henry Newton the exposures of the " Potsdam " sandstone will be seen 



