Geology . glomerate in the Arikaree formation of western Nebraska. (Darton, U. S. Geol. Surv.) the former range. Miocene deposits are known as far north as theFrancis River, and also on the Porcupine branch of the Yukon; buterosion rather than deposition was the dominant process in Alaska,so far as present data show. Igneous activity during the Miocene. — The wide-spread igneousactivity which began with the close of the Cretaceous and continued, folios of the U. S. Geol. Surv. Both sedimentary and igneous formations are repre-sented. 1 Dawson, G. M., Trans. Royal Soc. of Canada, 1890. THE MIOC
Geology . glomerate in the Arikaree formation of western Nebraska. (Darton, U. S. Geol. Surv.) the former range. Miocene deposits are known as far north as theFrancis River, and also on the Porcupine branch of the Yukon; buterosion rather than deposition was the dominant process in Alaska,so far as present data show. Igneous activity during the Miocene. — The wide-spread igneousactivity which began with the close of the Cretaceous and continued, folios of the U. S. Geol. Surv. Both sedimentary and igneous formations are repre-sented. 1 Dawson, G. M., Trans. Royal Soc. of Canada, 1890. THE MIOCENE PERIOD. 271 at least intermittently, through the Eocene, made itself felt also inthe Miocene, and perhaps reached its maximum toward the end ofthat period. The frequent references in preceding pages to igneousmaterials in the sedimentary formations of the system give someidea of the extent of Miocene vulcanism. The eruptions were fromfissures as well as from volcanoes, and extensive sheets of lava as. Fig. 449.—Monument of Gering (Miocene) sandstone over Brule (Eocene) clay, westernNebraska. (Darton, U. S. Geol. Surv.) well as volcanic cones were formed, and intrusions as well as extru-sions were of frequent occurrence. Evidences of volcanic activityduring this period are found in nearly or quite every State west ofthe Rocky mountains. Among other centers of igneous activity maybe mentioned the basin of the Columbia l and the Yellowstone National 1 Landes, Wash. Geol. Surv., Vol. II, and Smith, G. O., Ellensburg folio, U. SGeol. Surv. QEOLOQY. Park,1 where evidences of Miocene volcanic activity are to be seenon all hands. Locally,2 forests were buried by the volcanic ejecta,
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