Vol. 4] Knopf-Thelen. — Geology of Mineral King. 



241 



resemblance is misleading and merely fortuitous. Under the 

 microscope it is observed that plagioclase is not abundant, and 

 that the potash feldspar is to a great extent Lutergrown with 

 albite in microperthitie fashion. Quartz is unexpectedly present 

 in no inconsiderable amount, though relegated to interstitial 

 habit. The ferromagnesian minerals are sparingly represented 

 by biotite. 



Tourmaline Granite and Andesitic Dykes. — Cutting this 

 granite at various places are intrusive masses of tourmaline gran- 

 ite and dykes of feldspar porphyrites. The tourmaline is quite 

 abundant in the shape of radial aggregates, averaging a centi- 

 meter in diameter. A little plagioclase, orthoelase and quartz 

 form a rather fine grained ground throughout which the tour- 

 maline is distributed. The tourmaline spherulites are often 

 located in the center of areas of quartz of uniform optical orien- 

 tation. The porphyrites are characterized by an abundance of 

 white feldspar phenocrysts embedded in a dark gray-blue ground 

 of fine texture. Some biotite is noticeable, but is not conspic- 

 uous. In thin section the large feldspars display decided cor- 

 rosion phenomena, having had all the corners rounded off. They 

 correspond in composition to a calcic oligoclase. A not very 

 prolific generation of much smaller plagioclase lathes succeeded 

 them. They show no evidence of magmatic resorption, and are 

 not connected with the first generation by intermediate sized 

 feldspars. Some basaltic hornblende, for the most part strongly 

 chloritized, and biotite represent the ferromagnesian minerals. 

 Magnetite and apatite constitute the accessories. The ground- 

 mass, in which a little accessory quartz was noted, is holocrystal- 

 line and contains a great deal of chloritic matter scattered 

 throughout it. The microscopic diagnosis shows, therefore that 

 the porphyrites should more accurately be called andesites. 



The detailed petrography of the granitic rocks of the region 

 shows that in this portion of the Sierra Nevada there exist large 

 bodies of typical orthoelase granites, and even highly acid potash 

 granites. The time relations between the tonalite (grano-diorite ) 

 and granite magmas show that here the more acid preceded the 

 less acid (tonalite) intrusion. 



