572 DESCRIPTIVE GEOLOGY. 



roidal granular texture, resembling many varieties of marble. In the quartz- 

 grains may be seen under the microscope large numbers of fluid-inclusions. 

 Interstratified^ in the quartzite are thin seams or bands of black or steel- 

 gray quartzite, with the lines of contact between the two kinds sharply 

 defined. A precisely similar bed of white quartzite, interstratified in lime- 

 stone, with the same lithological features, both above and below it, has not 

 been recognized in any exposures of the great belts of limestones. It may, 

 however, represent some of the coarse yellowish-brown sandstones, or pos- 

 sibly some bed of arenaceous limestone, so frequently met with in other 

 localities, notably the brown sandstone bed in the limestone of Chokup 

 Pass, Diamond Range. - . 



The limestone has an even, fine-grained texture, and a gray to grayish- 

 black color. Disseminated through it are thin flakes and minute particles 

 of iron pyrites. A specimen taken from near the summit of Tenabo Peak 

 has been submitted to analysis by Mr. B. E. Brewster, with the following 

 result: 



Silica (chiefly sand) 19.899 



Silica (soluble) 1.088 



Iron pyrites 1.166 



Alumina and ferric oxide 1.088 



Lime 39.765 



Magnesia 2.799 



Carbonic acid 32.799 



Water and organic matter -. 1.059 



99.663 

 This gives : 



Carbonate of lime 67.545 



Carbonate of magnesia 5.878 



A minute quantity of the alkaline bases was detected, the spectro- 

 scope indicating the presence of both soda and potassa. 



No direct stratigraphical evidence was obtained as to the age of the 

 limestones, as they rest high up on the granite, cut off from all other sedi- 



