TOE POTSDAM AND TIIR IGNEOUS PEAKS. 103 



continuity cannot be doubted. The tract is so broken, cafioned, and intri- 

 cate, that it was ahnost an inn)ossibilit}' to pass tln-oug-li it at all, excejjt in 

 the immediate valley of some stream, and its examination was therefore 

 not so complete as could be desired; but the Potsdam, wherever seen, pre- 

 sents the same features that were observed in other parts of the Hills. 



In some of the valleys in the northern end of the Hills, where such ricli 

 deposits of placer j^old have been found since our exploration, the bed-rock 

 of the stream is Potsdam sandstone. The auriferous gravels ma}' in such 

 cases have been derived directly from the wearing down of the schists and 

 slates on the upper courses of the creeks, but it is more probable that they 

 came indirectly from the same source through the medium of the Pots- 

 dam. The streams which drain the Ai'chtcan area are concentrating the 

 gold by a method not very dissimilar from that of the Potsdam waves, and 

 it is reasonable to suppose that dirt which has been first rocked by the 

 waves and then sluiced by the creeks Avill hold its gold in a more concen- 

 trated condition than that which has had the benefit of but one process. 



Near Terry Peak, where there have been great outbursts of volcanic 

 matter, the Potsdam is changed in many places into a quartzite, and 

 the conglomerate in some places is so metamorphosed that a fracture takes 

 place as well through the quartz pebbles and bowlders composing it as in 

 any other manner, the cementing matrix being changed to n rock as hard 

 as the pebbles themselves. 



The Potsdam is exhibited also in an imperfect outcrop around the base 

 of Custer Peak, which seems to rest upon a foundation of that rock, and 

 thence we may follow the outcrop southward to Castle Creek, where we 

 took it up in the beginning of this narration. 



In the northwest end of the Hills the Potsdam is brought to view by 

 the volcanic uplifts at Warren and Crow Peaks. In each place its outcrop 

 encircles the igneous mass and inclines against it at a high angle, exhibit- 

 ing more or less metamorphism near the contact. About Warren Peaks 

 its color is dark brown ; in places it is a lamellar shale, and elsewhere it 

 becomes massive and hard, while locally there were found conglomerates 

 of brown sandstone pebbles resembling the Potsdam itself. About the 

 middle of the formation there occurs a thick stratum of shaly, brown or 



