424 Turner — Rocks and Minerals from Calif t 



or ma. 



manner inclusions of angite and pyrrliotite and occasional crys- 

 tals of the same primary brown amphibole that occurs in the 

 groundmass. Both the pyroxene and the amphibole grains 

 found in the amphibole phenocrysts are distinct inclusions, 

 with sharp contacts and with diverse orientation and cleavage. 

 The pyrrliotite appears to be original, as the amphibole about 

 it is not in the least altered. Some of the pyrrliotite was sepa- 

 rated by using a magnet on the powdered rock and tested with 

 HC1, giving off fumes of sulphureted hydrogen. The amphi- 

 bole of the groundmass in some cases shows sharp idiomorphic 

 outlines, and there is no evidence that it results from the 

 alteration of augite. 



Prof. G. H. Williams has described a somewhat similar rock 

 in this Journal (vol. xxxi, 1886, p. 40). Williams's rock is 

 largely made up of diallage with a little hypersthene and oli- 

 vine forming a groundmass in which are imbedded large idio- 

 morphic poikilitic amphiboles containing abundant grains of 

 the groundmass. The chief difference between the rock here 

 noted and Williams's rock is the complete absence of hypers- 

 thene and olivine and the presence of primary amphibole in 

 the groundmass and among the inclusions in the large amphi- 

 boles. 



This rock is one type of a very interesting series of basic 

 igneous rocks which are found in the foothills of Mariposa 

 County. Typical gabbros and diabases both with and without 

 olivine and amphibole-picrites represent different types of 

 these igneous rocks. They have not as yet. been carefully 

 investigated. 



3. A Quartz-alunite Rock* 



Southwest of Indian Gulch in Mariposa County, California, 

 are three sharp buttes known as the Tres Cerritos. The origi- 

 nal material of these buttes appears to have been chiefly meta- 

 augite-andesite tuffs (augite-porphyrite tuffs) and clay slate. 

 The Tres Cerritos lie immediately east of a considerable area of 

 clay slates which are pretty certainly of Jurassic age. They 

 are supposed to belong to the Mariposa formation. Not far 

 west of the buttes by the road to Merced these slates have 

 been metamorphosed into typical chiastolite-schists by a gran- 

 itic rock. 



The rocks of the Tres Cerritos have been subjected to solfa- 

 taric action and their angular outline is due to the hardness of 

 the altered rocks. The larger portion of these metamorphosed 

 rocks are light brown in color, highly siliceous, resembling 

 quartzite megascopically. 



The southeast butte, which was examined with the most 

 care, is composed chiefly of a greenstone-schist wdiich is a 



* nth Annual Report, U. S. G-eological Survey, Part I, p. 685. 



