Geology . Fig. 562.—Pressure ridge developed in fresh lavas. Jordan Craters, Ore. (Russell, U. S. Geol. Surv.) touched by erosion that there can be little doubt that they date froma time long subsequent to the beginning of the Quaternary period. THE PLEISTOCENE OR GLACIAL PERIOD. 479 Judged by the same criteria, there are lava-flows and cinder cones ofQuaternary age in New Mexico (Fig. 563)/ Colorado, Utah, Nevada,Oregon (Figs. 561 and 562), Idaho (Fig. 560),2 Washington,3 and atvarious points in the On many of them vegetation has hardlybegun to gain a foothold. Gilbert estimates tha
Geology . Fig. 562.—Pressure ridge developed in fresh lavas. Jordan Craters, Ore. (Russell, U. S. Geol. Surv.) touched by erosion that there can be little doubt that they date froma time long subsequent to the beginning of the Quaternary period. THE PLEISTOCENE OR GLACIAL PERIOD. 479 Judged by the same criteria, there are lava-flows and cinder cones ofQuaternary age in New Mexico (Fig. 563)/ Colorado, Utah, Nevada,Oregon (Figs. 561 and 562), Idaho (Fig. 560),2 Washington,3 and atvarious points in the On many of them vegetation has hardlybegun to gain a foothold. Gilbert estimates that of 250 lava fieldsobserved in these states, 15% are of Pleistocene age, and of 350 vol-canic cones in the same states, 60% are considered to be ash is inter bedded with loess at various points in eastern. Fig. 563.—Edge of malpais (lava), Tularosa Desert, White Oak, N. M. (Hill, U. S. Geol. Surv.) Washington and Oregon,6 and overlies glacial moraines in some partsof Alaska. Glacier Peak, Washington, is the remnant of a volcanoformed after the base-leveling (Pliocene) of the Cascade Mountainregion, and probably after the elevation of the base-leveled Rainier dates from about the same time. 1 Tarr, Am. Nat., Vol. 25, pp. 524-527, 1891. 2 Nampa, ida., folio, U. S. Geol. Surv.; also Russell, Bull. 217, U. S. Geol. Surv. 3 Tacoma and Ellensburg, Wash., folios, U. S. Geol. Surv. 4 See Bidwell Bar, Colfax, Downieville, Lassen Peak, Pyramid Peak, and Truckee,Cal., folios, U. S. Geol. Surv. 5 Mono. I, U. S. Geol. Surv., pp. 323-337. 6 Jour, of Geol., Vol. IX, p. 730. 1 Russell, 20th Ann. Rept. U. S. Geol. Surv., Pt. II, p. 134. 480 QEOLOQY. rgneoilS rock lias occasionally had a significant influence on modern vegetation, without regard to the age of the lava it-self. The unwooded
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