610 PEOF. J. P. IDDINGS OBT EXTKTJS1VE [Nov. 1 896, 



parable with those of the Andes in size, as well as in the nature of 

 their material. The later eruptions from these volcanoes were 

 quieter outflows of lava, which probably took place after the 

 position of the volcanic conduits had become more stationary. 

 Erosion having carried away the upper parts of these great cones, 

 the remaining portions are almost wholly made up of breccia, in 

 places 4000 feet thick. 



The last of the great eruptions were equally violent, though of a 

 different kind. They were gigantic fissure-eruptions that flooded 

 the region west of the chain of denuded volcanoes with massive 

 streams of lava that rose high up on the flanks of the surrounding 

 mountains, and then flowed toward the south-west, leaving, when 

 cooled and after erosion had somewhat reduced the surface of the 

 stream, a vast sheet of lava at least 1000 feet thick in most places, 

 and over 2000 feet thick in some parts. This was followed by 

 other outflows from fissures that flooded the region for hundreds of 

 miles to the south-west and west, and closed the period of activity. 

 Within the Yellowstone Park the earliest breccias were accumulated 

 in Eocene time, and the great bulk of the Absaroka volcanoes in 

 Miocene; while the great fissure-eruptions just mentioned took 

 place in the Pliocene. 



With this crude sketch before us, let me attempt to fill in, as it 

 were, some of the details. To this end I have drawn an outline- 

 map of the territory involved, indicating only the drainage and the 

 surface-distribution of the volcanic rocks. The chain of volcanoes 

 which is now represented by the tuff-breccias and included lava 

 sheets, dykes, stocks, etc., extends without interruption from imme- 

 diately south of Bozeman, Montana, at about lat. 45° 30', south- 

 ward 40 miles in the Gallatin Mountains, to a short distance within 

 the northern boundary of the Yellowstone Park. 1 It extends east- 

 ward about 50 miles through the southern half of the Snowy 

 Mountains along the northern boundary of the Yellowstone Park, 

 to the head-waters of Clark's Pork River. Thence southward in 

 Wyoming it forms the Absaroka 2 range along the eastern boundary 

 of the Park, 3 and continues beyond the head-waters of the Yellow- 

 stone River to the head-waters of Wind River, where it forms the 

 rugged peaks about Togwotee Pass and the Washakee Needles, 4 

 terminating at about lat. 43° 30' N., or 105 miles from the last- 

 mentioned bend. The length of the chain is about 170 miles. In 

 the north it is at present 12 or 15 miles wide, and in the Absaroka 

 range from 25 to 50 miles wide. The volume of igneous rock that 

 was erupted to form this range must have been enormous, when 



1 Iddings and Weed, ' Livingston Folio, Geologic Atlas of the United 

 States,' no. 1, Washington, 1894. 



2 Misprinted ' Assaroka ' in the accompanying map (PI. XXIX). 



3 Hague, A., Compte-rendu 5eme Sess. Int. Geol. Oongr. Washington, 1891, 

 p. 339 ; also Iddings, ' A dissected Volcano of Oandall Basin, Wyoming,' 

 Journ. Geol. vol. i. (1893) p. 606. 



4 St. John, O., Report in Hayden's 12tfi Ann. Rep. U.S. Geol. & Geogr. 

 Surv. of the Territories, pt. i. p. 260, W ashington, 1883. 



