The Yellowstone national park, historical and descriptive, illustrated with maps, views and portraits . ias; the later ofrhyolite, while all along there were smaller flows ofbasaltic lavas. The andesitic eruptions played their prin-cipal part in the up-building of the mountains. Over thegreater part of the xlbsaroka and Gallatin Eanges theolder granite and sedimentary rocks were buried beneaththe lava, and the modern form of these mountains is thatwhich time has wrought out from these igneous rocks. These volcanic outbursts were evidently not so much ofthe character of molten lava as in later
The Yellowstone national park, historical and descriptive, illustrated with maps, views and portraits . ias; the later ofrhyolite, while all along there were smaller flows ofbasaltic lavas. The andesitic eruptions played their prin-cipal part in the up-building of the mountains. Over thegreater part of the xlbsaroka and Gallatin Eanges theolder granite and sedimentary rocks were buried beneaththe lava, and the modern form of these mountains is thatwhich time has wrought out from these igneous rocks. These volcanic outbursts were evidently not so much ofthe character of molten lava as in later times. In manyplaces the heat was not sufficient to consume organic sub-stances, the forms of which have remained intact to thepresent time. The material was apparently not liquidenouoh to spread itself about like a lake, but insteadbanked up in the near neighborhood of eruption and thuspromoted the building up of the mountains. It seems alsoto have been of a character that yielded readily to theagencies of erosion. There were several craters from which these lavas issued i tj __ cj _o __ m: < *. o Th£ Traverti>-e Rocks. GEOLOGICAL HISTORY OF THE PARK. 193 —two or more in the Absaroka Range, one in the GallatinRange, and two, which interest us more, in the interior ofthe Park, Mt. Washburn and Mt. Sheridan. No one canEtand on the summit of ^It. Wasliburn and look down uponthe forest-covered amphitheater that forms the watershedof Tower Creek, without feeling instinctively that he isstanding on the rim of an ancient crater, which was oncea seething caldron of molten lava, but is now eJothed in agarb of beauty by the gentler agencies of nature. In the process of time the eruptive material from thesevolcanoes showed a marked change in character. The laterflows were mainly of rhyolite. It is this rock that makesthe Park plateau what it is to-day. It was of a moreliquid character than the early outflows, and spread itselfall over the country, filling up its depressions and
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