104 GEOLOGY OF THE BLACK HILLS. 



gray, argillaceous sandstone full of fucoid impressions, and where the rock 

 is more massive the holes of Scolithus are very abundant, fairly honey- 

 combing it. These borings (?) are particularly numerous near the upper 

 part of the sandstone. The base of the formation was concealed at the 

 point of measurement, but the exposed section contains fully 350 feet. 

 North of the peaks, at the head of the north branch of the Redwater, 

 the Potsdam is full of the impressions of fucoids or sea-weeds, expecially 

 near the base of the upper third of the formation. These fossil impres- 

 sions are not found on merely isolated specimens, but over a large region 

 almost every fragment of rock that may be picked up is found impressed, 

 generally in relief, with the branching form of sea-weeds. 



Even in the better studied and most fossiliferous localities of the forma- 

 tion fucoids occur but sparingly, and the great quantity found here and 

 elsewhere in the Potsdam of the Black Hills indicates an abundance of 

 vegetable life in the old Potsdam sea that has heretofore not been fully 

 realized 



Before leaving the subject of the Potsdam a few words will be devoted 

 to the manner of its formation and the physical character of the region at 

 the time, so far as they have been inferred from a study of the structure 

 and relations of the rocks. 



The whole character of the Potsdam rocks indicates them to be shore 

 and shallow water deposits. The conglomerate, containing large bowlders 

 of the Archaean rocks, could only have been produced by the immediate 

 action of waves on a shore line, breaking against a reef or coast-wall, and 

 rolling the fragments torn from it to and fro on the shore until they became 

 smooth and rounded as we now find them. The sand of the sandstones is 

 the result of the more complete trituration of the same materials by the 

 shore waves, and being lighter and more easily moved by water currents it 

 occupied a position some distance from the immediate shore line, but still 

 at a nearness depending upon the size of the grains and their specific 

 gravity, the coarser and heavier being deposited nearer and the smaller 

 and lighter farther from the beach line. The heavy conglomerate is found 

 only at the very base of the formation and in position nearest the axial line 



