POTSDAM SANDSTONE. 285 



chyte the Potsdam sandstone rests against the slopes of the outer circle of 

 hills. Its lower layers are so metamorphosed by the heat and chemical 

 solutions accompanying the upheaval, that they merge imperceptibly into 

 the igneous rock, until strata which are probabl} r altered sandstone cannot 

 be distinguished from coarsely-crystalline trachyte. Near the base of the 

 Potsdam formation, between strata of unmistakable sandstone, a mass of 

 feldspar-trachyte was observed, which had in the eruption escaped between 

 the layers and slightly altered the adjacent surfaces of the inclosing rock. 

 But more frequently the metamorphism seems to have been the result of 

 the action of heated solutions forced between the layers of the sandstone, 

 altering the surface of the fracture and transforming the sedimentary rock 

 into a feldspar porphyry. 



Layers of the soft brown sandstone extending horizontally for some 

 distance had been altered in this manner, the rock being shot full of gray 

 feldspar crystals, from one-fourth to one-half an inch long, irregularly dis- 

 tributed through it. 



On the west slope of Warren Peaks a layer of blue slate occurred, 

 inclosed in the trachyte, but conformable in its bedding to well-marked 

 Potsdam about 50 feet above it, and was probably formed by the action of 

 heat on a layer of clay-slate in the base of the Potsdam. Here this for- 

 mation is full of fine specimens of the large fucoids so characteristic of the 

 Potsdam sandstone in the Black Hills, and great masses of the upper layers 

 of the rocks are perforated by the borings of marine worms. These fossils 

 are found unaltered within a few feet of the trachyte, and would seem to 

 show that the heat accompanying the upheaval of the range and the intru- 

 sion of the igneous rocks was not very intense. At the south of the peaks 

 the whole thickness of the Potsdam sandstone has been so metamorphosed 

 that it cannot be recognized, except by the evidences of an indistinct stratified 

 structure, and its position under the Carboniferous limestone. The altered 

 sandstone on that side is a quartzite more or less porphyritic, with feldspar 

 crystals merging in all gradations into a ciystalline rock resembling tra- 

 chyte. Even here the metamorphism seems to be more from the action of 

 heated chemical solutions permeating the rock than from the direct action 

 of heat, producing a semi-fusion of the Potsdam strata. The Carboniferous 



