POST-JURASSIC ROCKS OF NEVADA 



39 



spars. The granite at Oak Spring in the Belted Range contains 

 phenocrysts of feldspar, quartz, and biotite which reach maximum 

 lengths of 4 inches, ^ inch, and i inch respectively. The feldspar 

 phenocrysts are locally sufficiently abundant to form one-third of the 

 rock mass. The microscopic character of the granular siUceous 

 rocks are tabulated in Table I, and those of the corresponding por- 

 phyries in Table 11. 



From the east to the west edge of the area under consideration 

 the rocks in the main become progressively less siHceous and the 

 quartz-monzonite of the Panamint Range approaches in composition 

 the granodiorite of the Sierra Nevada. In a broad way then the 

 magmas in the various centers of igneous activity were less and less 

 siHceous from meridian 116 west to the Sierra Nevada. There is 

 also a sUght increase in the soda content of the rocks from the eastern 

 border of the area to the western. 



The soda-syenite and soda-syenite porphyry which form a small 

 mass intrusive in Pennsylvanian limestone ii| miles southeast of 

 Tin Mountain (Panamint Range) are worthy of more detailed des- 

 cription. The soda-syenite is characterized by abrupt and great 

 changes in granularity and in the relative abundance of the constit- 

 uent minerals. The most common type is a coarse to medium- 

 grained gray rock composed of predominant gray with some pink 

 feldspar, subordinate greenish-black amphibole or pyroxene, and 

 biotite. Many of the feldspars have good crystal outlines and in the 

 more porphyritic facies the abundant feldspar laths which have 

 lengths of i| inches are ahgned in flow orientation. The rock, next 

 to the Hmestone, is very fine grained. Epidote has developed at 

 the expense of the ferromagnesian minerals in all facies. Under the 

 microscope this rock proves to be a soda-syenite or nordmarkite of 

 uneven hypidiomorphic granular texture. AlkaH feldspars are the 

 predominant constituents and include the species orthoclase, micro- 

 perthite, and anorthoclase. With these is a Httle oHgoclase. The 

 alkaH feldspars form rude tabular crystals, many of which are 

 twinned according to the Carlsbad law. Augite sometimes occurs 

 in columns, or anhedra of augite, green hornblende, quartz, biotite, 

 and yellowish-brown garnet may He between these tabular forms. 

 The augite shows characters approaching aegirite-augite. The 



