240 



University of California Publications. 



[Geology 



Microscopically, the quartz-mica-diorite is a medium grained 

 granular rock, the tabular character of whose plagioclase feld- 

 spars is often apparent, even textured, and of gray color. Its 

 weathered surface is characteristically of blue gray tint, and 

 usually pitted, due to the atmospheric removal of the hornblende. 

 Under the microscope it is seen that the total ferromagnesian 

 content is relatively small and comprises biotite and hornblende, 

 the mica commonly predominating. Plagioclase is present in 

 large amount and is noteworthy from the strong tendency it 

 displays to assert its idiomorphism. Its habit is tabular, and 

 the feldspars almost invariably consist of Carlsbad twins stri- 

 ated according to the albite law. An accurate determination is 

 therefore readily possible, and shows that the feldspar corre- 

 sponds to andesine Ab 5 An 3 . A variable amount of orthoclase 

 occurs, though usually in strict subordination to the soda-lime- 

 feldspar. It is always allotriomorphic, and occasionally twinned 

 according to the Carlsbad law. Strain shadows are observable, 

 sometimes culminating in a microcline structure. A not incon- 

 siderable amount of quartz is distributed throughout the body 

 of rock so as to form an irregular mesostasis in which the tab- 

 ular feldspars lie embedded. It is probably owing to this inter- 

 stitial character that the quartz owes its macroscopic ineonspieu- 

 ousness. A few minute occurrences of graphic intergrowths 

 were observed. The accessories are magnetite, titanite, zircon, 

 and apatite somewhat abundantly. 



The characterization of this rock shows that it is closely 

 allied to the granodiorites, in the sense in which the term grano- 

 diorite has been most frequently employed ("quartz-mica-dio- 

 rite carrying orthoclase"). It also accords exactly with the 

 tonalite of vom Rath, to which, in the nomenclature of Rosen- 

 busch, it would unhesitatingly be placed. The dark colored seg- 

 regations so common in the granites are conspicuously absent in 

 the tonalite. 



Closely resembling the tonalite of Monarch Creek is a body 

 of granite apparently intrusive into the hornblende granitite 

 on the Wet Meadows trail. More careful inspection shows that 

 the feldspars are largely unstriated and that the macroscopic 



