Geology . 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 vol
Geology . 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,. Fig. 450.—Petrified tree-trunks, Yellowstone National Park.(Iddings, U. S. Geol. Surv.) and in favorable situations their trunks were petrified (Fig. 450).Great areas of the sedimentary beds of the period are concealed by 1 See western folios, U. S. Geol. Surv., notably the Yellowstone National Parkfolio. Most of the folios showing Neocene formations show volcanic rocks of Neoceneage. 2 Yellowstone National Park folio. THE MIOCENE PERIOD. 273 the lavas, but the extrusions were by no means confined to the areaswhere Miocene sedimentation had been in progress. While igneous activity has been in progress interruptedly sincethe earliest known times, the record of few periods of geological his-tory shows such extraordinary extrusions of lava as those of the Ter-tiary. The exact stage of the Tertiary at which the great lava sheetsof the west were extruded has not been determined in all cases; butthe lavas of at least a considerable part of 200,000 or 300,000 squaremiles of lava-covered co
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