14 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



great Avail is formed of nearly horizontal beds of limestone (Flathead), upon 

 which rests the great mass of intrusive andesite-porphyry whose southern 

 extension has already been noted, and over which arches the distinctly 

 bedded limestone that forms the eastern part and the summit of Antler 

 Peak. This is the great intrusion termed by W. H. Holmes the "Indian 

 Creek laccolite," 1 whose mass 'forms the greater part of Antler Peak, the 

 bold summit north of Indian Creek, and its extension westward to the 

 slopes of Three River Peak. The structure and topography of this part of 

 the ridge are clearly shown in the sketch by Mr. Holmes. 2 In the middle 

 of the ridge the overlying strata are absent, but at the western end of the 

 ridge they recur, completing the westward-dipping limb of the arch, as 

 shown in PI. Ill, fig. 2. 



This dome-shaped body of intrusive rock is a cross section of the 

 double sheet met with in the mountains south. In Antler Ridge it attains 

 its maximum thickness, and appears as one massive body with a thin layer 

 of shale or limestone inclosed near its middle, which is indicated in Mr. 

 Holmes's sketch. It is, however, not absolutely continuous. No doubt it 

 is the thin edge of the limestone wedge that split the intrusive mass in two 

 as it was forced southward. 



An examination of the limestone underlying the laccolith shows that 

 the prominent cliff, 75 to 100 feet high, is formed of the nodular limestones 

 of the Flathead formation corresponding- to the lower limestone belt of the 

 Crowfoot section. These beds are more fully noted in the section of the 

 sedimentary rocks of Antler Peak. Within the lowest micaceous shale 

 beneath the laccolith there occurs a layer of white, lithoidal, igneous rock, 

 50 feet thick, and evidentl}^ a horizontal sheet, which is again exposed at 

 about the same horizon 2 miles farther west. Petrographically it resembles 

 the rock of Mount Holmes, of which it is probably an offshoot. 



The contact between the lower massive limestone and the bottom of 

 the laccolith is plainly exposed in places. The limestone exhibits little or 

 no metamorphism, there being only a slight lightening of its color along 

 the immediate contact. The crude columnar jointing of the massive lime- 

 stone may have been the result of baking by the laccolithic mass, but it is 

 not pronounced ; however, it may easily be mistaken at a distance for the 



1 Twelfth Ann. Rept. U. S. Geol. and Geog. Surv. Terr, (for 1878), Part II, Washington, 1883. 



2 Op. cit.. PI. XIII, p. 21. 



