MOUNT HOLMES BYSMALITH. 19 



southwest, and through Mount Holmes and Trilobite Point. These cross 

 the contact plane between the bysmalith and the strata with the inclosed 

 laccolith. 



Owing to the crystalline character of the rock constituting the bys- 

 malith, there is little doubt that it solidified beneath a covering of strata. 

 The crystals are larger than those forming the mass of the Indian Creek 

 laccolith. A possible reconstruction of the position of the strata after the 

 intrusion is given in PI. V, fig 4, in which all of the formations up to 

 the top of the coal-bearing Laramie are represented — a total thickness of 

 9,000 feet, 



The upper parts of the mountains into which this intrusion has been 

 carved are barren and rocky above 9,000 feet, with comparatively smooth 

 slopes covered with loose fragments of porphyry. The peaks are pointed 

 in some cases and rounded in others, as may be seen from PI. IV. The 

 rock is very uniform in appearance throughout the entire extent of the 

 mass. It is light gray to white, aphanitic to fine crystalline, with a slightly 

 porphyritic structure in part. It shows small flakes of biotite and indistinct 

 phenocrysts of feldspar. It is massive, with irregular joint cracks, and 

 weathers into angular blocks and slabs. Its megascopical characters are 

 quite uniform throughout the greater part of the mass, which varies slightly 

 in grain. But near the margin of the body the rock becomes denser and 

 more aphanitic, showing a broad banding parallel to the walls of contact 

 with the surrounding rocks. These walls are nearly vertical in the moun- 

 tain west of The Dome, on the saddle east of Mount Holmes, and also on 

 that of Echo Peak. In all cases examined, the neighboring limestones dip 

 away at angles of 40° to 55°, and the adjacent andesite-porphyry has been 

 crushed and dislocated. It is reddened and in places is filled with veins 

 and apophyses from the dacite-porphyry, which is clearly proved to be an 

 intrusion subsequent to that of the Indian Creek laccolith. Its western 

 border in contact with the crystalline schists is obscured by de'bris, being 

 located at the' base of the mountains. These relations are shown in PI. V. 



The eruption appears to have taken place along the fault line that lies 

 west of Three River Peak. There seems to be no break in the continuity of 

 the bysmalith mass, and this fact indicates that it was intruded at one time. 

 The vertical displacement of a mass of rock 2£ miles long and 2 miles wide, by 

 what appears to have been a single act, is remarkable. The petrographical 

 character of the rock is that of an intrusive, not a surficial, body, hence, 



