42 SYDNEY H. BALL 



(Nevada) ranges, and Louderback' states that granite intrudes 

 Triassic rocks in the Humbolt (Nevada) range. 



Later students of Nevada geology beheve with King^ that the 

 mountain ranges of the western part of the state and the Sierra Nevada 

 were formed simukaneously at the close of the Jurassic. The granite 

 intrusion was probably a relatively late event in this period of deforma- 

 tion, an inference supported by the relatively unmashed condition of 

 the granite. The erosion, prior to the outpouring of the earhest 

 Tertiary lavas, of the thick covering of superincumbent rocks, which 

 must have been present in order to produce the granular texture of the 

 plutonic rocks also points to the post- Jurassic or very early Cretaceous 

 age of the igneous rocks. Such an age determination is in harmony 

 with that of the granodiorite of the Sierra Nevada. That mountain 

 system and the ranges of western Nevada have had a closely parallel 

 history. 



While these granular siHceous rocks are thus believed to have 

 been contemporaneous in a broad way, their consoHdation unquestion- 

 ably occurred at sHghtly different times. This is indicated not alone 

 from the relation between the pegmatite of the quartz-monzonite 

 and the soda-syenite of the Panamint Range, but also from the 

 wide lithologic variety in the granular rocks of the Silver Peak Range 

 (see column 7, Table I, and columns i, 2, 6, and 7, Table II). There 

 indeed the granular rocks may have solidified from wholly separate 

 magmatic basins. 



The conclusion as to the post- Jurassic age of these rocks is in 

 accord with that of Spurr, for the granites of Silver Peak^ and Gold- 

 field,4 Nevada, and that of Louderback^ for similar rocks in the 

 Humbolt Range, Nevada. 



APLITE 



The granitoid rocks of most of the areas are cut by narrow dikes 

 of aphte, a fine-grained rock of white or pink color. In composition 



1 George D. Louderback, Bulletin Geological Society of America, Vol. XV, p. 318. 



2 Clarence King, U. S. Geological Explor. 40th Parallel, Vol. I, 1878, p. 759. 



3 J. E. Spurr, Bi-monthly Bull. Amer. Inst, of Min. Engrs., 1905, No. 5, p. 955; 

 also Professional Paper No. 55, U. S. Geological Survey, pp. 25, 26. 



4 J. E. Spurr, Bull. 260, U. S. Geological Survey, 1905, p. 133. 



s G. D. Louderback, Bull. Geol. Soc. of Amer., Vol. XV, 1904, p. 336. 



