MADISON EAXGE. 57 



sedimentarv strata and their tilting northward, followed by extensive 

 erosion at the south prior to the extravasation of the andesitic breccia. 



We have already pointed out the connection between the intrusion of 

 the Holmes bvsmalith and the fault traversing- the eastern end of Crow- 

 foot Ridge, noting their probable contemporaneity. The intrusion of the 

 Holmes mass must have been followed by extensive erosion before the 

 crystalline schists were exposed at the level they now occupy relative to 

 the. Holmes mass, after which erosion the andesitic breccias were thrown 

 upon them. This separates the eruption of the Holmes bvsmalith and that 

 of the andesitic breccias by a very considerable length of time. No definite 

 time relation has been made out, however, between the two great intrusive 

 bodies at the southern end of the Gallatin Range — Indian Creek laccolith 

 and Holmes bysmalith — and the more complex intrusion of Gray Peak 

 and the associated sheets of andesite-porphyry in the northern part of the 

 range. Though separated by only a small distance, there is no structural 

 feature which connects their intrusions in point of time, except the general 

 fact that the} r are all much older than the eruptions that centered at Electric 

 Peak. 



EASTERX FLAXK OF THE MADISOX RAXGE. 



In the extreme northwestern corner of the Yellowstone Park there is a 

 small area of mountainous country that is part of the eastern flank of the 

 Madison Range. This area lies wholly within the Montana portion of the 

 reservation. The Gallatin River has cut a narrow valley across this tract, 

 exposing folded strata, in which the same sedimentary series seen in the 

 Gallatin Range is developed, the lowest rocks belonging to the Cambrian 

 and the highest being of Colorado Cretaceous ag-e. These strata are flexed 

 about a laccolith of andesite-porphyiy. 



This mountain area is terminated on the south by the northern end of 

 the rhyolite plateau, whose lavas cover the southeastern flanks of the high 

 mountain east of the Gallatin River and also occur in small isolated patches 

 upon the mountain slopes to the north and west. 



Topographically this little tract consists of parts of four distinct moun- 

 tain masses. The largest lies east of the Gallatin and is embraced between 

 that river and Fan Creek. This block and that north of it, and the fiat- 

 topped mountain west of the Gallatin River, are all parts of the lacco- 

 lithic uplift, which has been cut through by the river. The east bank 



