SANDSTONE DIKES OF UTE PASS. 141 



the Potsdam. The basal member of the Potsdam, it will 

 be remembered, is a white to gray sandstone more or less 

 interstratified or blended with the prevailing reddish 

 brown variety. It is not only calcareous or argillaceous, 

 but it contains insufficient iron oxide for its thorough 

 cementation, and has very clearly never been exposed to 

 volcanic influences. We can, therefore, readily conceive 

 that it remained unconsolidated for a Ions: time after the 

 lithifaction of the overlying beds. The dike rock is abso- 

 lutely indifferent to the changes in the character of the 

 neighboring sedimentary formations, showing no appre- 

 ciable variation as, in succession, from Manitou southeast 

 to Cheyenne Canon, the Potsdam, Silurian, Carbonifer- 

 ous, Triassic and Dakota beds abut against or border the 

 great fault. 



The close association of the dikes, throughout the en- 

 tire belt, with the great displacement, and their unvary- 

 ing lithological similarity to the Potsdam sandstone, have 

 suggested to me that the dikes probably date from the 

 formation of the Ute fault ; that the fault probably dates 

 from the time when the Potsdam beds, which are still at 

 the base in part of a more or less friable character, were 

 imperfectly consolidated and covered the entire region ; 

 that the fault, as is likely to be the case with a great dis- 

 placement, was not simple, but that a moderate breadth 

 of the granite and overlying formations was traversed by 

 a series of parallel fissures ; and that the dikes resulted 

 from the sinking of the Potsdam sandstone and sand into 

 the fault-fissures. Such local subsidences of the friable 

 sandstone would naturally be attended by a more or less 

 complete obliteration of the bedding. That the structure 

 of the Potsdam beds has been locally effiiced under shear- 

 ing and compressive or plicating movements can be seen 

 at a point on the east side of Ute Pass a few rods below 



KSSEX INST. BULLETIN, VOL. XXVn 13* 



