260 REPORT UNITED STATES GEOLOGICAL SURVEY. 



might be cited, simply showing that the conditions necessary for the 

 formation of that product which we call basalt existed long before the 

 advent of the Tertiary epoch. 



For Colorado we can accept, as a rule, a Tertiary age for basalt. 

 Several exceptions seem to occur, however. We have in basalt the vol- 

 canic eruptive which connects the prehistoric eruptions of our continent 

 with those of the present day. In previous pages reference has been 

 made to a statement by Dr. Peale, suggesting that the time of the erup- 

 tions might rather be measured by several hundred years than by long 

 periods of time. All geologists who have explored the Western States 

 and Territories speak of the remarkably fresh appearance of a large 

 quantity of the ejected material. In speaking of Utah volca,nics Dr. 

 Engelmann* says: "The mineralogical character of the rocks seems to 

 indicate that their formation began prior to the Tertiary period and 

 continued to the present era." Mr. Gilbert gives a description of some 

 very " fresh " specimens of lava and scoria from the Lower Sevier Valley 

 in Utah.t 



In New Mexico a number of localities occur where the lava is simi- 

 larly fresh. Dr. Newberry in discussing Mount Taylor| says : " The 

 lava-streams of its latest eruptions present precisely the same appear- 

 ance as those of Vesuvius when but just cooled." His examinations of 

 the San Francisco Mountains revealed to him the same state of affairs. 

 If possible the condition here is even more striking. Newberry states :§ 

 " Some of the currents of lava which have flowed down the sides of the 

 San Francisco Mountains belong so entirely to the present epoch that 

 they have dispossessed still running streams from their beds, and now 

 occupy their places in a congealed flood which seems but just arrested 

 in its flow." 



As we advance from what maj'^ be considered as interior land toward 

 the Pacific Ocean, or, with other words, as we leave the Eocky Mount- 

 tains and reach the coast-ranges, we find still more positive evidence ot 

 the recenc3^ ot volcanic eruptions. Coincident with the increasing quan- 

 tity of points of eruption and with the younger age, we find that the 

 characteristic of "massive" eruption is gradually disappearing, and 

 " volcanic " eruptions take their place. This necessarily results in the 

 formation of crater-cones. Mr. Gilbert || has found three perfectly dis- 

 tinct craters at Ice Spring, near Fillmore, in Utah.^j 



Among the mountains of Washington Territory and Oregon, as well 

 as in California, we find more than one still showing signs of volcanic 

 action. Prominent among these is Mount Shasta. Professor Whitney 

 finds** near the summit of this peak "the remains of the edge of the 

 crater," where " there are several orifices from which steam and sul- 

 phurous gases are constantly escaping, and around which is a con- 

 siderable deposit of sulphur, some of it being handsomely crystallized." 

 It may, in truth, be called a small solfatara. Similar conditions occur 

 near Mono Lake, about which Whitney says: ft "To the north lies a 

 group of islets ;"...." there is every indication that the volcanic 

 action has ceased at a very recent period, or, rather, that it has not yet 

 fully died out." .... " This portion of the island is of hard, black 



* Rep. Expl. across the Great Basin, by Capt. J. H. Simpson, in 1859, Wash., 1876, 

 p. 307. 

 t Expl. and Surv. west 100th Mer., 1875, vol. iii, p. 566. > 

 t Rep. Expl. Exp., 1859, J. N. Macomb, 1876, p. 61. 



$ Rep. upon the Colorado River of the West, J. C. Ives, 1861, part iii, p. 66. 

 II Expl. and Surv. West 100th Mer. 1875, vol. iii, p. 138. 

 IT Compare : Proc. Am. Ass. Adv. Sci., Hartford, 1874, p. 29. 

 ** Geol. Surv. Calif., vol. i, 1865, p. 340. 

 ttlbid.,p. 453. 



