POST-JURASSIC ROCKS OF NEVADA 45 



The phenocrysts of orthoclase and microperthite predominate over 

 those of plagioclase (oligoclase and oligoclase-andesine). Altered 

 biotite phenocrysts are constantly present while quartz crystals, much 

 corroded, appear in some thin sections. Zircon and magnetite are 

 accessory minerals. 



This quartz-monzonite porphyry which evidently contains con- 

 siderable soda, is younger than the granite which it cuts in the area 

 four miles west of AlkaU Springs and pebbles of it are included in the 

 Siebert lake beds (Miocene). 



DIORITE PORPHYRY AND DIORITE 



Dikes of diorite porphyry and a few intrusive masses of diorite 

 occur in the Silver Peak, Panamint, Grapevine, and Cactus ranges, 

 the Gold Mountain and Slate ridges and the Bullfrog, Mount Jackson, 

 and Lone Mountain hills. These rocks are hence confined to the 

 western half of the area under consideration. In composition they 

 range from acid diorite approaching granodiorite to a quartz-augite 

 diorite of ophitic texture. Brown hornblende characterizes the more 

 basic types and fragments of serpentine found in the Lone Mountain 

 foothills may be altered forms of still more basic phases. 



The diorite-porphyry and diorite are younger than the Paleo- 

 zoic rocks and the igneous rocks already described, but nowhere 

 were they observed cutting Tertiary lavas. Pebbles occur in the 

 Siebert lake beds (Miocene) and the rock is probably of pre-Tertiary 

 age. 



The quartz-monzonite porphyry and the diorite porphyry and 

 diorite, which are the youngest of the pre-Tertiary igneous rocks, 

 occur only in comparatively small masses. They are probably com- 

 plementary differentiation products of a magma residual from the 

 soHdification of the granular siliceous rocks. 



