ABSTRACTS 40/ 



of Pliocene age. Still later the recent basalts, the last of the igneous 

 extrusions, poured out over the rhyolite along the ridges of the 

 plateau A generalized vertical section accompanies the text, showing 

 the order of succession of the extrusive flows, from the earliest out- 

 bursts to the final dying out of eruptive energy. It is shown that long- 

 continued currents of heated waters and acid vapors have acted as 

 powerful agents in decomposing the igneous rocks of the plateau, and 

 date back to Pliocene time ; at least they were active before glacial ice 

 covered the country. Hot springs, geysers, and solfataras are closely 

 associated with the rhyolite, and in fact thermal activity is confined 

 almost exclusivelv to areas of this rock. 



The illustrations relate mainly to the occurrence of both active and 

 dormant geysers and hot springs or some phase of volcanic geology. 

 The Grand Canyon, well shown in the illustration, is a profound 

 gorge cut in the Pliocene rhyolite, the brilliant coloring being due to 

 the action of thermal waters. 



Geologic Atlas of the United States. Folio 24, Three Forks, Montana, 



/( 



This folio by Dr. A. C. Peale, consists of five pages of text, a topo- 

 graphic sheet (scale i : 250,000), a sheet of areal geology, one of 

 economic geology, one of structure sections, and one giving a gener- 

 alized columnar section for the district. 



The area covered comprises the square degree which lies between 

 the meridians 1 11° and 112° and the parallels 45° and 46 , m the 

 southwestern, mountainous portion of Montana, and includes 3354 

 square miles. In the extreme southeast corner the Yellowstone 

 National Park barely falls within the area. The folio derives its name 

 from the valley in which the Jefferson, Gallatin, and Madison rivers 

 unite to form the Missouri. The "Three Forks" valley is important 

 from an historic standpoint, as being the point which Lewis and Clark 

 reached in July, 1805, when they named the three confluent branches 



of the Missouri. , 



The text begins with a general description of the geography and 

 topography of the region, and then takes up the general geology. 

 The oldest rocks in the region are the crystalline schists and gneisses, 

 designated as of Archc^an age, which in pre-Cambrian time formed a 

 land niass comprising nearly all the area included in the map. While 



