104 JOSEPH P. IDDINGS 



torsional shearing stresses of great magnitude, as is shown by the 

 rapid variation in the amount of displacement along the planes 

 of faulting. These fractures have occurred in nearly parallel 

 planes, and also in a nearly rectangular system, and resulted in 

 faults and joints. 



The fractured and dislocated rocks were eroded to a very great 

 extent at a very early period. Thus from a large area the entire 

 sedimentary covering, io,ooo feet or more in thickness, was 

 removed from the underlying crystalline schist before Eocene 

 times, and the surface of the country presented an irregular topog- 

 raphy not greatly different in character from that of the pres- 

 ent day'. Upon an irregular surface of crystalline schists in the 

 present valley of the Yellowstone River, just south of the Living- 

 ston quadrangle, there rest horizontal beds of Eocene volcanic 

 tuff with ancient tree trunks in vertical position. Erosion has 

 reduced the rocks in this vicinity to nearly the same surface at 

 successive periods, as is shown by the occurrence of surface extru- 

 sive lavas of very different ages in close proximity. The direc- 

 tion of the drainage in this locality has undoubtedly shifted 

 repeatedly during this long lapse of time. 



Since volcanic activity commenced, fracturing and faulting 

 have taken place at intervals through the Tertiary period. The 

 profoundest faulting and erosion took place before Eocene times 

 and the extravasation of Eocene tuffs. This was followed by 

 great accumulations of Miocene tuffs and lavas, which were 

 fractured and dislocated along the faults in Cinnabar and Reese 

 Creeks, and elsewhere. These were greatly eroded in late Ter- 

 tiary time before the eruptions of Pliocene rhyolite which has 

 been faulted in its time. 



In the faulting of the Electric Peak and Sepulchre Mountain 

 blocks there was unquestionably a displacement along the line of 

 the earliest northwest-southeast fault against which these blocks 

 terminate proving a recurrence of fracturing and displacement 

 along old lines of jointing and faulting. And while there is little 

 or no evidence in this region that successive faulting has often 

 taken place at widely remote geological periods along the same 

 lines, it seems probable that profound jointing may establish 



