Geology . ark.(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 know
Geology . ark.(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 country in the western part of the United States : Ki ^<. Fig. 451.—Sections of petrified logs, near Holbrook, Ariz. Age of beds not known. issued during the Miocene period, or during the time of crustal defor-mation which brought it to a close. The volcanic activity of the time was not restricted to the Cor-dilleran system, but affected also the Antillean system of CentralAmerica and the West Indies,1 and the Andean system of SouthAmerica. Close of the Miocene.—During the Miocene, there appears to havebeen more or less crustal movement throughout the Cordilleran warpings of the surface seem to have been in progress, while 1 Hill, Geology of Jamaica. Reviewed in Jour, of Geol., Vol. VII. 274 GEOLOGY. faulting, vulcanism, and gradation all produced changes in the physi-ography of tin1 west. Locally, as in the Santa Cruz mountains of Cali-fornia, there were pronounced orogenic movements in the courseof (he period, but toward its close crustal movements Beem to havebeen general. At this time pronounced deformative move
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