POST-JURASSIC ROCKS OF NEVADA 41 



which were among the first to sohdify from the magma and also in 

 anhedra between the tabular feldspars, this titanite being among 

 the last minerals to separate from the magma. In other phases 

 of the pegmatite partial crystals of ferromagnesian minerals are in- 

 closed in a coarse crystalline aggregate of feldspar. Microscopic 

 examination shows that orthoclase and microperthite are so predomi- 

 nant over plagioclase in this rock that it has the composition of a 

 syenite. A little quartz is present and the ferromagnesian minerals 

 include augite, oHvine, brown hornblende, and biotite. Augite in 

 unusually large crystals and magnetite in grains also occur. 

 In this rock there are associated the two minerals, quartz 

 and olivine, which at one time were believed never to occur 

 together. 



Weathering. — Southwestern Nevada has an arid climate and con- 

 sequently the granitoid rocks weather into a soil composed of frag- 

 ments of the constituent minerals which are practically fresh. This 

 mechanical disintegration, largely dependent upon stresses due to 

 changes of temperature, is so rapid that broad basins of granitic soil 

 extend up into the hills. Through these detrital embayments low 

 domical outcrops of granite protrude and it is only in the highest 

 peaks that exposures are conspicuous. 



Age. — From the uniform amount of mashing which these rocks 

 have suffered and from the similarity in the geological history of the 

 various ranges, it is evident that all of these rocks are approximately 

 of the same age. They intrude the Paleozoic rocks as batholiths, 

 stocks, sheets, and dikes, where the contacts can be observed, and 

 fragments of them are included in Tertiary lavas. The youngest 

 Paleozoic rocks in the area are of Pennsylvanian age and these are 

 intruded by granular igneous rocks in the Belted and Panamint 

 ranges. The earliest lavas occur in the Stonewall Mountains and 

 these rocks, which are of very late Cretaceous or early Eocene age,^ 

 contain fragments of granite. The granular rocks are, therefore, 

 of post- Carboniferous and pre-Tertiary age. Spurr^ found rocks of 

 similar composition cutting Triassic strata in the Pilot and Excelsior 



I Sydney H. Ball, Bulletin jo8 U. S. Geological Survey, p. 2i3- 

 ^ J. E. Spurr, Bulletin 208, U. S. Geological Survey, pp. 103, 109. 



