56 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 3D SER. 



parallel to the bedding. Many of these quartzite sheets 

 appear to be wholly free from mica, the surface of the sheet 

 glistening from the minute quartz crystals composing the 

 rock. Other specimens, in cross-section, appear to be of 

 clear quartz, but when viewed in the plane of the bedding 

 numerous minute scales of muscovite are seen scattered 

 over the surface. In all fractured surfaces these flakes are 

 seen to be arranged with their planes parallel to the plane 

 of the bedding. There are all gradations between this and 

 specimens in which the mica is the most prominent mineral. 



The layers which form the partings of the quartzite beds 

 are quite dark, varying from a dark gray to a yellowish or 

 reddish color due to iron stain. They are finely schistose 

 and readily flake off; are quite soft and have usually a 

 smooth, silvery surface. Even where this silvery luster is 

 not at first apparent it may easily be made out with a lens. 

 The layers appear to be composed of mica or its decompo- 

 sition products. Tested chemically the mineral shows the 

 presence of a large amount of alumina, a little iron, no lime 

 and a little magnesia, besides giving a decided flame reac- 

 tion for potassium. The optical characters of the flakes 

 could not be determined, owing to their want of transpar- 

 ency. 



A considerable proportion of the rocks of the west end 

 have much the appearance of gray sandstone to the unaided 

 eye, though with a lens they are seen to be composed 

 largely of this micaceous material, with minute lenses or 

 grains of the quartzite scattered through it. These mica- 

 schists occur indiscriminately with the rocks which are 

 more properly quartzites, and occasionally lens -shaped 

 masses of the quartzite are found in such areas. There 

 are all gradations between these rocks in which the mica is 

 predominant and those in which the quartz predominates. 



Besides the micaceous partings of the quartzites there 

 were found at a number of points partings of blue amphi- 

 bole, having frequently a silky luster. This amphibole also 

 occurs in larger masses in a schistose condition. The oc- 

 currences of this rock were not mapped, but they are found 



