12 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



amount of schists and some ancient and metamorphosed intrusive bodies; 

 the more detailed account of which is given in Part I of this monograph. 



Along the eastern base of the mountains the gneiss forms prominent 

 exposures, constituting the end of the long southeast spur of Trilobite Point 

 and forming bold escarpments on either side of the valley northeast of 

 Mount Holmes, below the level of the glacially carved lake basins. The 

 gneiss forms a bench extending along the eastern base of The Dome, the 

 exposure having a height of 300 or 400 feet, and occurring across the valley 

 of Indian Creek, where it is last seen to form a low, wooded hill on the 

 northern side of the creek. 



Immediately overlying the gneiss around three sides of Trilobite Point 

 is the lower sheet of andesite-porphyry, a light-gray aphanitic rock with 

 porphyritical crystals of feldspar, hornblende, and biotite. The sheet is 

 between 200 and 300 feet thick and occupies the horizon of the Flathead 

 shales, being overlain by 150 to 200 feet of Cambrian, Flathead limestone 

 in nearly horizontal beds. Above the limestone is another sheet of andes- 

 ite-porphyry, from 100 to 200 feet thick, which in turn is topped by the 

 Upper Cambrian shale and trilobite-bearing limestone. The upper surface 

 of the uppermost sheet of andesite-porphyry is quite irregularly defined, 

 and the overlying limestone is traversed by small dikes and veins of igne- 

 ous rock, that are in part offshoots from the lighter-colored igneous mass of 

 Mount Holmes. The basal (lower) sheet of andesite-porphyry is thicker 

 at the northern side and thinner at the southern side of the mountain. 



At the saddle on the ridge connecting Trilobite Point with Mount 

 Holmes, near the contact of the rocks just described with the igneous rock 

 of the latter mountain, the beds of limestone and andesite-porphyry are 

 turned up to an angle of about 45°, dipping eastward, away from the 

 Holmes mass. The strata are greatly fractured and are penetrated by many 

 small bodies of the Holmes rock. 



THE DOME. 



The Dome is a mountain summit northeast of Mount Holmes and con- 

 nected with it by a low ridge. It is separated from Antler Peak and the 

 northern portion of the range by the wide and deep valley of Indian Creek. 

 The mountain is largely formed of andesite-porphyry, an extension of the 

 Indian Creek laccolith, which rests upon crystalline schists and is capped 

 by Cambrian limestones forming the summit of the peak. A thin belt of 



