52 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



The andesite-porphyries of this section occur as sheets intruded along 

 the planes of bedding of the sedimentary rocks. With one exception 

 intrusion is regular and does not break across the bedding planes of the 

 shale. The dip and strike of the beds vary in passing upward from Gardi- 

 ner River to the summit of the peak. In many cases this change of dip 

 is apparent in the exposure, but in most cases the outcrop is not suffi- 

 ciently extensive to make the flexure apparent. 



The long southwestern spur of Electric Peak, which has already been 

 noted, shows sheet after sheet of andesite-porphyry cutting across the crest 

 of the ridge and intruded in the dark shales of the Colorado formation, 

 the beds dipping NE. about 10°. Three different bodies of andesite- 

 porphyry have broken through one another in the high point southwest of 

 the peak. One of these intrusions may be traced eastward along the 

 southern slope of the mountain for some distance. The western slope of 

 the mountain shows the sedimentary and igneous layers in strong relief, 

 and they may be distinguished at a distance. The inclination of these 

 ledges is about 10° N., corresponding to the greater dip, which is to the 

 northeast. The sheets of igneous rock are seen to follow the bedding planes 

 of the strata for considerable distance. Only one sheet was seen break- 

 ing up across the strata and proceeding along the higher horizon. The 

 direction of this uprising is from south toward the north, and this occur- 

 rence, together with observations made on the eastern side of the moun- 

 tain, showing a similar rise from the west to the east and a thinning out 

 of the sheets in the same direction, indicates that these intrusive masses 

 were injected from the southwest— that is, probably from the center occu- 

 pied by the Gray Peak bysmalith. The intrusive sheets vary from a few 

 feet to a hundred or more feet in thickness. The rocks differ slightly in 

 petrographical character and will be described more fully in Chapter II. 

 The peak itself is formed of the soft shales and thinly bedded sandstones 

 belono'ine' to the Colorado and Montana formations. These rocks are 

 penetrated by a number of intrusive dikes, of andesite-porphyry, and the 

 sedimentary series is much altered by the great intrusion of igneous rock 

 forming the volcanic core east of the peak. On the summit of the peak the 

 normal sandstones and shales are altered to slates and quartzites, the rocks 

 being much shattered by joints and breaking readily into short, angular 

 debris. The beds dip N. 10° to 20°. 



