OUTLINE OF THIS VOLUME. 



Chapter I. The Gallatin Mountains, extending 18 miles within the boundary of the Yellowstone 

 National Park, consist of sedimentary strata ranging from the Cambrian, through the Silurian, 

 Devonian, Carbouiferous, and Juratrias, to the Laramie of the Cretaceous. These sedimentary rocks 

 have been uplifted by forces acting from the southwest. They dip northeast, and have been folded 

 to a slight extent transverse to the strike. Subsequently they have been strongly faulted. The dis- 

 location at the close of the Laramie was accompanied by intrusions of igneous magmas in several 

 large laccolithic bodies and in numerous sheets, and in the vicinity of Electric Peak by dikes. Erosion 

 has uncovered crystalline schists at the southern and southwestern end of the range, and has laid baie 

 exposures of all the sedimentary and igneous rocks. Finally, glaciation has modified the topography 

 in a striking manner. The structural relations of the sedimentary and igneous rocks are illustrated 

 by a number of geological sections. 



Chapter II. This chapter treats almost exclusively of the intrusive rocks of the Gallatin Moun- 

 tains. They are mainly fine-grained and aphanitic masses, in most occurrences porphyritic and 

 andesitic in character. The large bodies differ from one another somewhat in composition, and vary 

 slightly in texture, in different parts of the rock bodies. In one intrusive sheet there has been a pro- 

 nounced differentiation by the settling of phenocrysts of augite. 



Chapter III. Electric Peak and Sepulchre Mountain are described as parts of a Tertiary volcano 

 which were faulted across the conduit, the amount of vertical displacement having been more than 

 5 000 feet. The deeper portions of the mountains, consisting of sedimentary strata intersected by 

 dikes, sheets, and the stock or conduit of the volcano, have been brought to the surface, as shown in 

 the mass of Electric Peak. The ejected breccias and lava flows, together with the upper portion of 

 the conduit, constitute Sepulchre Mountain. Lavas which are andesites in the latter mass are diorites 

 and porphyries in the former. Rocks with like chemical composition are found to have different 

 mineral composition according as they are crystallized into phanerocrystalliue diorites or into apha- 

 nitic andesites. 



Chapter IV. The northern end of the Teton Range extends but a short distance within the Yellow- 

 stone National Park. It consists of a nucleus of crystalline schists and gneisses overlain by Paleozoic 

 and Mesozoic strata flexed in an anticline with northward-dipping axis and faulted to a slight extent. 

 Birch Hills, a few miles to the north, are an outlier of the range. Upon greatly eroded strata basic 

 breccias were thrown out, and after these had undergone fresh erosion vast flows of rhyolite covered 

 the country and now form a part of the plateau of the Park, beneath which the northern extremity 

 of the Teton Range is hidden. 



Chapter V. The country described in this chapter embraces a mountainous area irregular in 

 outline and of great diversity of form. It is situated in the southern part of the Park and the Yel. 

 lowstone Park Forest Reservation. It consists of a number of ridges trending northwesterly and 

 southeasterly, formed for the most part of Mesozoic rocks. The older sedimentary rocks are exposed, 

 but the ridges are essentially made up of sandstones of Cretaceous age. The irregular outline of the 

 mountains is due to the rhyolites of the Park Plateau that abut against the slopes of the upturned 

 beds. The principal physical features of the region are Wildcat Peak and Huckleberry Mountain, 

 Bobcat Ridge, Big Game Ridge, Chicken Ridge, Two Ocean Plateau, and the gorge of Snake River. 

 West of Huckleberry Mountain occur several exposures of dacite surrounded by rhyolite. They are 

 among the few outcrops of dacite known in the Park, and are apparently older than the rhyolite. In 

 the gorge of Snake River the Madison limestones, Teton sandstones, and the Ellis limestones and 



