A. Hague — Igneous Rocks of the Yellowstone Park. 445 



Art. L. — The Age of the Igneous Rocks of the Yellow- 

 stone National Park ; by Arnold Hague, of the U. S. 

 Geological Survey. 



The region embraced by the Yellowstone National Park 

 and the adjoining country to the north and east, has been a 

 center of great volcanic activity. By far the greater part of 

 the surrounding mountains, and the entire park plateau, have 

 been built up by the pouring out of vast accumulations of vol- 

 canic material. The region, throughout Tertiary time, was 

 undoubtedly the most active center of eruptive energy to be 

 found in the northern Rocky Mountains. 



Under the authorization of the U. S. Geological Survey, 

 aided by an able corps of assistants, I have carefully studied 

 the geological history of volcanic eruptions in the Park coun- 

 try, and the order of succession of the different masses of lavas 

 which make up this enormous body of igneous rocks. To a 

 proper understanding of the geological relations of these vol- 

 canic lavas to the earlier crystalline and sedimentary rocks, as 

 well as their relations to the pre-existing mountain ranges, a 

 brief sketch of the geology of the country seems necessary. 



In all the mountain ranges which surround and shut in the 

 park, at least a nucleus of highly crystalline rocks of Archean 

 age is exposed. The Tetons, to the south, consist mainly of 

 an Archean mass, which towers high above all later rock for- 

 mations. In the Absaroka range, stretching along the entire 

 east side of the park and formed mainly of igneous rocks, 

 granites and schists are exposed at the northern end, which 

 soon pass between the later lavas. The Snowy range, which 

 shuts in the park on the north, is largely made up of Archean 

 schists, gneisses and granites, associated with the more recent 

 outbursts of lava. In the Gallatin range, on the west, a body 

 of crumpled gneisses and schists form the nucleus around 

 which have been built up a complex mountain structure, in 

 which the volcanic rocks play an important part in the struc- 

 tural features of the mountains. Resting unconformably upon 

 these Archean bodies, which formed either a broad continental 

 mass, or a group of closely related islands, occurs a great thick- 

 ness of sedimentary beds, made up in greater part of material 

 derived by the slow processes of denudation of the earlier con- 

 tinental land surfaces. These sediments, thus slowly deposited, 

 built up during Paleozoic and Mesozoic times a series of sand- 

 stones, limestones, and shales conformable from base to summit. 

 'The basal beds, resting directly upon the Archean rocks, con- 

 sist largely of siliceous material, passing gradually over into a 



