820 INDEX TO THE STEATIGKAPHY OF NOETH AMERICA. 



contemporaneous with monzonite porphyry, quartz monzonite porphyry, hornblende-biotite 

 latite, and biotite andesite of other ranges. These rocks are in part intrusive masses and in 

 part flows. Prior to the succeeding extrusion of rhyolite these rocks * * * were eroded, 

 unconformities being observed in the Kawich, Amargosa, and Cactus ranges, in the Bullfrog 

 Hills, and in Shoshone and Skull mountains. Then followed an extrusion of rhyolite, with 

 minor siliceous latites and dacites, which was attended by insignificant basalt flows. This 

 period of extrusion of rhyolitic lavas, the most important in this portion of the Great Basin, 

 was not, however, strictly contemporaneous over the whole area. * * * This rhyolite is 

 everywhere separated from the Siebert lake beds by a marked erosional unconformity, and 

 in the Kawich and possibly in the Cactus Range it is separated by an erosional unconformity 

 from the succeeding andesites. The next younger igneous rocks * * * are basic andes- 

 ites. These two rocks were found in contact only in the area covered by the Goldfield special 

 map, and here Mr. F. L; Ransome found dacites intrusive into the andesites. The succeeding 

 igneous rocks are rhyolites and sihceous latites and dacites which occur only in the Goldfield 

 and Southern Klondike liills and the Silver Peak Range. In the latter two localities at least 

 these rocks are interbedded with the Siebert lake beds without erosional unconformity. 



Thick masses of sediment occur in the majority of the ranges of the area and on litho- 

 logic and stratigraphic grounds are correlated with the Siebert lake beds of Miocene age at 

 Tonopah described by Spurr. These tuflaceous sandstones and conglomerates, largely com- 

 posed of rhyohtic material, reach an observed maximum thickness (in the Amargosa Range) 

 of 1,150 feet. * * * 



The succession of the lavas of the Great Basin has recently been treated at length by 

 Spurr." His succession for the petrographic province of the Great Basin and that of the writer 

 for the portion of that province here treated are practically identical. Both assign the first 

 rhyohte to the end of Cretaceous and to Eocene time. It is believed, however, from the length 

 of time indicated by the complex history of extrusion and erosion intervening between this 

 rhyolite and the deposition of the Siebert lake beds, that the monzonites and acidic andesites 

 may well also be in part Eocene, while the second rhyolite is beHeved to be largely of early 

 Miocene age. The andesite and dacite are also of comparatively early Miocene age, while the 

 third rhyolite covers the middle and late Miocene and early PUocene. The basalts range in 

 age from late Miocene to Pleistocene, the major extrusions occurring in late Phocene and Pleis- 

 tocene time. As to the genetic relations of the magmas, the writer is wholly in accord with 

 the views of Spurr in the article already cited. He beheves that the Tertiary lavas of the 

 Great Basin are the representatives of two complete cycles of the differentiation of a magma 

 of medium composition into acidic and basic lavas and that probably the end of a still earlier 

 cycle is also represented. 



K 10. NORTHERN CALIFORNIA. 



Miocene and Pliocene deposits occur in the old valleys of upper Sacramento 

 and Trinity rivers, on the southeastern slope of the Klamath Mountains and west 

 of Lassen Peak. They comprise auriferous gravels, fresh-water sediments with 

 some coal, allied to the lone formation, and tuffs. These deposits have been 

 described by Diller,"^' ^■^®'' Anderson,^* and Hershey.*^*^ 



K 11-12. NEVADA, UTAH, AND IDAHO. 



Only the later Tertiary (Pliocene) is represented in the northern and eastern 

 parts of the Great Basin. King ^"'' states : 



Along the western base of the Wasatch a portion of the terrace country, rising to 700 or 

 800 feet above the level of the lake, is composed of loose, friable Tertiaries, carrying very 



a Spurr, J. E., Jour. Geology, vol. 8, 1900, pp. 621-646. 



