XVI OUTLINE OF THIS VOLUME. 



shales are well shown. The Snake River hot springs are situated near the contact of the rhyolite 

 with the Carhoniferous limestone, the lime of the travertine being derived from the Madison lime- 

 stones. The incrustations around the springs resemble the travertine deposits found at the Mammoth 

 Hot Springs. The characteristic and limited Wolverine flora, of Laramie age, occurs near the base 

 of Pinyon Peak. The conglomerate of Pinyon Peak, a striking physical feature of the region, is 

 described as overlying unconformably the Laramie sandstones, and evidence is given showing that 

 the conglomerate probably belongs to Eocene time, as it underlies the basic breccia of the Absaroka 

 Range. The impressive gorge of Outlet Canyon cuts a deep passage completely through Chicken Ridge. 

 The interesting feature of the canyon is that it at one time served as the discharge for the waters 

 of Yellowstone Lake. This sheet of water, which now flows northward and drains to the Atlantic 

 through Yellowstone Canyon, formerly discharged into Snake River and thence to the Pacific. 

 Two Ocean Plateau shuts in the sedimentary ridges on the east. The plateau, which rises 10,000 

 feet above sea level, forms a part of the Absaroka range and is made up of similar volcanic breccias 



and silts. 



Chapter VI. The extreme southern end of the Snowy Range forms the northeast corner of the 



Park. The crystalline core of the range forms a broad, plateau-like summit, bordered by sedimentary 

 rocks of Paleozoic age, which along the south slope dip gently away from it toward the Park. The 

 highest peaks, together with extensive areas, are formed of andesitic breccias, but erosion has cut 

 through them and exposed the underlying limestones, showing that the volcanic rocks rest upon a 

 very uneven and rugged surface. Detailed sections of the Paleozoic sedimentary rocks from the Flat- 

 head formation to the Madison limestone are given, but the igneous rocks are described iu other 



chapters. 



Chapter VII. The Miocene volcano of Crandall Basin built itself upon a ridge of eroded Pale- 

 ozoic rocks which dip toward the southwest from the crystalline schists of the Beartooth Range. 

 Beneath the volcano are remnants of Eocene breccias and lava flows. The volcano consisted of basic 

 andesitic breccias topped by basalt flows and traversed by dikes that radiated from the stock or core 

 which ^vas the conduit beneath the crater. While bedded breccias characterize the outer portions of 

 the volcano, chaotic imbedded breccias form the central portion. Comparison with modern active 

 volcanoes indicates that the Crandall volcano rose to about 13,400 feet above its limestone floor. The 

 phanerocrystalline rocks within the core are gabbros and diorites, approaching monzonites in part, 

 and are chemically like the basalts and andesites of the breccias, dikes, and flows, but differ from 

 them in mineral composition. They also are parts of the volcano and are properly volcanic rocks. 



Chapter VIII. The Absaroka Range consists largely of volcanic breccias, with subordinate 

 amounts of massive flows or intrusive bodies. This chapter presents a petrographic treatment of those 

 igneous rocks which lie within the limits of the Yellowstone Park, and their discussion is confined to an 

 account of their field occurrence and distribution and a systematic description of their mineralogical 

 characteristics and composition. The earliest accumulations occur at the northern end of the range 

 and are made up of early acid breccias found in disconnected remnants beneath early basic breccias. 

 They consist mainly of hornblende-audesite and hornblende-mica-andesite. The early basic breccias 

 are pyroxene-andesite, passing upward into the massive basalt flows. Upon the latter were thrown 

 the late acid breccias, similar in composition and appearance to the early acid breccia. This passes 

 upward into late basic breccia, consisting of basic andesites with less basalt than is associated with 

 the early basic breccia. The late basic breccia forms the southern portion of the range within the 

 Yellowstone Park and also Two Ocean Plateau. At Sylvan Pass and in its vicinity it is traversed by 

 dikes of andesite and a few of diorite. Remnants of surficial flows of massive andesite form the 

 summits of Mount Stevenson, Mount Doane, Colter Peak, and several prominent mountains south of 

 Sylvan Pass. 



