16 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the generally low dip recurring again farther north. At Bighorn Pass the 

 laccolith thins out and reaches its western limit. 



In Three River Peak the porphyry is also overlain by limestone in 

 nearly horizontal beds with slight westerly dip, the uppermost strata 

 belonging to the Madison limestones of the Carboniferous. 



Along the western boundary of the laccolith, between Three River Peak 

 and Bighorn Pass at the head of Gallatin River Valley, the limestone strata 

 dip W. 45°, becoming less inclined farther west, where they encounter a 

 fault trending west of north, which brings them against gneiss and steeply 

 tilted Cambrian beds. Unfortunately the strata bordering the laccolith on 

 the north, along the bottom of Panther Creek Valley, are covered with 

 loose material from the mountain slopes; hence the position of the rocks 

 adjacent to the laccolith on the north was not discovered. In several 

 places the porphyry has broken up through the overlying limestone. 



Without entering too minutely into the petrographical character of 

 the laccolith of andesite-porphyry, which will be described in detail in 

 Chapter II, it may be well to mention some of its general characteristics. 

 The andesite-porphyry is a light-gray to whitish aphanitic rock with many 

 small phenocrysts of feldspar and fewer of biotite and hornblende. It 

 forms massive outcrops intersected by joints in all directions, and the rock 

 splits, upon Aveathering, into sherdy, angular fragments. In only one place 

 was columnar jointing noted — on the southeast spur of The Dome. In the 

 central part of the laccolith the groundmass of the rock has a crystalline 

 texture, though extremely fine grained. Near the margin the rock grows 

 denser and darker colored. The same is true where the sheets become 

 thinner toward the south. The extent of this mass in exposure is shown 

 on the map, and its relation to the surrounding rocks is given in the 

 cross sections. 



MOUNT HOLMES BYSMALITH. 



A great mass of igneous rock, 3 miles long and 2 miles wide, forms 

 Mount Holmes and the ridge north to the summit of the peak west of The 

 Dome, and extends across the head of Indian Creek and constitutes the 

 chain of four peaks west of this creek and south of Three River Peak. This 

 great body of igneous rock breaks up through the sedimentary strata as a 



