A FRACTURE VALLEY SYSTEM 103 



Yellowstone and the Boulder Rivers. That these directions are 

 parallel to existing fractures is shown by the trend of dikes of 

 igneous rock that traverse this part of the country. A number 

 of dikes occur in Crow Indian Reservation on the Boulder River. 

 They trend northwest almost exactly parallel to the small streams 

 in this vicinity. Two longer dikes occur on Gage Creek, trend- 

 ing in the same direction. In the valley of Shields River there 

 are dikes having a northeast-southwest trend. The Crazy Mount- 

 ains are filled with innumerable dikes radiating in all directions 

 from the core of igneous rocks lying north of the Livingston quad- 

 rangle. That portion of the mountains within the limits of the 

 map is traversed by dikes trending' north-south in the middle of 

 the group, on the east side trending east of south, and on the 

 west side trending west of south, but not parallel to the drainage 

 channels in the foothills. Apparently these fractures are immedi- 

 ately connected with the intrusive core of the mountains, and have 

 not extended into the surrounding sedimentary strata for long dis- 

 tances. The system of fractures indicated by the drainage is prob- 

 ably more profound and was produced by deeper-seated forces. 



There are dikes and lines of intrusive rocks parallel to neigh- 

 boring drainage channels in the vicinity of Haystack Peak, Emi- 

 grant Peak, and at the west base of Sheep Mountain. 



Besides the drainage that may have been controlled or initi- 

 ated by fracture lines, there are within the area covered by the 

 map excellent examples of channels that have followed softer 

 rocks, where the minor topographic features of the country con- 

 form to the position and character of the sedimentary strata, 

 and where fracture and faulting appear to have had no influence 

 on the drainage. It is the larger valleys and dominant channels 

 that exhibit the general relationship between drainage and 

 fracture. 



That the drainage system of this quadrangle is closely related 

 to the structural features of the rock formations will not appear 

 surprising when the geological history of the region since Lara- 

 mie times is taken into account. 



The dislocation of the crystalline schists and sedimentary 

 strata that followed the deposition of the Laramie resulted from 



