550 KNOPF : ANDALUSITE MASS 



The andalusite mass is bordered on the north and northeast 

 sides by schistose volcanic porphyry and breccia characterized 

 by widely scattered phenocrysts of plagioclase. East of the 

 main andalusite body are outcrops of other considerable masses 

 of similar andalusite rock, but more mixed with other minerals, 

 and associated with these in thick beds are some brilliant white 

 rocks, soft and composed almost wholly of flakes of muscovite. 



The age of these rocks is in all probability pre-Cambrian. 

 This conclusion is based on what is known of the structure and 

 stratigraphy of the portion of the Inyo Range to the south. 2 

 The sedimentary rocks to the south range from pre-Cambrian 

 to Triassic, all the intervening systems being represented ex- 

 cept the Silurian. They aggregate in thickness more than 

 36,000 feet, but in all this great assemblage there are no rocks 

 that resemble those associated with the andalusite mass. They 

 are in general less metamorphosed, except perhaps the oldest 

 pre-Cambrian rocks recognized; they are not notably conglom- 

 eratic; and they are of marine origin. In the northern part 

 of the Bishop quadrangle, which is only 8 miles south of the 

 andalusite deposit, Cambrian and pre-Cambrian strata, in- 

 truded by Cretaceous granite, make up the range. The pre- 

 Cambrian consists of dolomite, sandstones, and dolomitic lime- 

 stones; of these the massive Reed dolomite, 2000 feet thick, 

 is the most readily recognizable formation. Rocks of this 

 kind do not occur near the andalusite mass, and it is probable, 

 therefore, that the rocks inclosing it are of still older pre-Cam- 

 brian age than those in the Bishop quadrangle. 



THE ANDALUSITE MASS 



The andalusite mass is well exposed, owing to the rugged- 

 ness of the region. It is certainly more than 300 feet wide at 

 its maximum and is 500 feet long; and, as shown by the deep 



2 Knopf, Adolph. A geologic reconnaissance of the Inyo Range and the east 

 slope oj the southern Sierra Nevada, with a section on The stratigraphy of the Inyo 

 Range, by Edwin Kirk. U. S. Geol. Survey Prof. Paper 110 (in course of publi- 

 cation). 



