80 GEOLOGY OF THE BLACK HILLS. 



not merely the time in which the slates and schists and quartzites were 

 deposited as clay and sand, but also the time in which they were subjected 

 to the vast displacement and degradation whereby their truncated edges 

 were finally prepared to receive the later sediments. The intrusion of the 

 granite took place before those changes were greatly advanced, and possi- 

 bly before they were begun, for it occurred in the presence of a heat and 

 a pressure that could not have existed close to the surface of the earth. 



SECTION IV. 

 THE SILURIAN. 



THE POTSDAM SANDSTONE. 



The members of the great Silurian system, which form such volumi- 

 nous chapters in the geological history of the territory east of the Missis- 

 sippi River, attaining along the Appalachian Mountains a thickness of no 

 less than 38,000 feet, thin out westward. In the Rocky Mountain region 

 but few of the groups have been recognized, and those occupy but a very 

 subordinate position when compared with the other members of the Rocky 

 Mountain rock series. Of the lower Silurian formations, the Primordial, 

 Canadian, and Trenton, there is found in the Black Hills only the 

 Primordial, and of the Primordial no group but the Potsdam, while the 

 different formations of the Upper Silurian — the Niagara, Salina, Helder- 

 berg, and Oriskany — are entirely absent. In other portions of the Rocky 

 Mountains beds have been recognized by different geologists as belonging 

 to the Canadian, Trenton, and Niagara formations, but they are always 

 inferior in thickness and extent to the more recent formations. 



In a few localities in New Brunswick, Nova Scota, and the Eastern 

 States a series of fossiliferous beds, known as the Acadian or St. John's 

 group, have been found underlying the Potsdam, but the latter is the 

 first in order of time of the widespread, unchanged, fossiliferous strata of 

 the American geological column. In the great area it is known to underlie, 

 in its great persistency, and in the uniformity of its character, the Potsdam 



