224 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



DIKES. 



A still more noticeable feature of the central portion of the district is 

 the occurrence of dikes which form prominent walls of rock traversing the 

 country in all directions. They are specially numerous in the region of 

 chaotic breccia, but are not confined to it. Owing to the importance 

 attached to the position and trend of the dikes, it has been thought advisable 

 to describe them in considerable detail, in order to assure the reader that 

 their location on the map is the result of careful observation. 



In the southwestern part of the district they are most noticeable cross- 

 ing the spurs at the head of Miller Creek, where they were observed by 

 Superintendent Norris. They are nearly all parallel, and trend northeast 

 and southwest, a few having a more westerly direction. Eight of them cut 

 the slope of the amphitheater at the northern base of Parker Peak. They 

 are from 3 to 8 feet wide, and often rise from 3 to 15 or 20 feet' above the 

 ground. They are nearly vertical and parallel, almost straight, with slight 

 curves and sometimes sharp bends, and may be traced by the eye across 

 the spurs in a north-northeasterly direction. The long spur south of the 

 branch of Miller Creek is crossed by several dikes having a northeast trend. 

 They consist of the same kinds of rock as two dikes cutting the summit of 

 Saddle Mountain, which also have the same trend and appear to be continua- 

 tions of these dikes. 



Hoodoo Mountain and the ridge between Lamar River and Timber 

 Creek are traversed by numerous dikes, some of which trend about N. 10° 

 W. and N., and others S. 30° E. and SE. Most of these dikes are from 3 

 to 8 feet wide. 



Proceeding from Hoodoo Mountain northwest and north, one finds 

 that the divide between Miller and Timber creeks is cut by dikes trending 

 northeast; and the high ridge through Indian Peak, and the Peak itself, 

 are traversed by dikes trending N. 30° E. and N. 20° E. In this peak, as 

 at Saddle Mountain, the dikes cut the massive flows of basalt, which were 

 therefore some distance beneath the surface of the volcano when the dikes 

 were injected. 



The valley of Timber Creek is covered with a heavy growth of pines 

 and firs, which obscures the geology and prevents the location of dikes 

 except by closer study than there was opportunity to bestow on it. But 

 the long narrow ridge north of this branch is bare of timber on its crest, 



