THE PLEISTOCENE OR GLACIAL PERIOD. 



479 



Judged by the same criteria, there are lava-flows and cinder cones of 

 Quaternary age in New Mexico (Fig. 563)/ Colorado, Utah, Nevada, 

 Oregon (Figs. 561 and 562), Idaho (Fig. 560), 2 Washington, 3 and at 

 various points in the Sierras. 4 On many of them vegetation has hardly 

 begun to gain a foothold. Gilbert estimates that of 250 lava fields 

 observed in these states, 15% are of Pleistocene age, and of 350 vol- 

 canic cones in the same states, 60% are considered to be Pleistocene. 5 

 Volcanic ash is interbedded 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 parts 

 of Alaska. Glacier Peak, Washington, is the remnant of a volcano 

 formed after the base-leveling (Pliocene) of the Cascade Mountain 

 region, and probably after the elevation of the base-leveled tract. 7 

 Mount 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, 

 (al.. folios, U. S. Geol. Surv. 



5 Mono. I, U. S. Geol. Surv., pp. 323-337. 



6 Jour, of Geol., Vol. IX, p. 730. 



; Russell, 20th Ann. Rept. U. S. Geol. Surv., Pt. II, p. 134. 



