720 DESCRIPTIVE GEOLOGY. 



more tliickly bedded quartzites, below whicb the porpliyroids come in well 

 developed. On the long spurs in the region of Buena Vista and Cotton- 

 wood Canons, the compact marls pass down into argillites, made up largely 

 of admixtures of claye}^ mud and quartz sand, with interstratified bands of 

 grit, impure jaspers, and chert, all well exposed, exhibiting a great variety 

 of dull earthy colors. 



A characteristic variety of argillite, possessing an almost impalpable 

 texture and homogeneous base, found on the northern ridge of Cotton- 

 wood Canon, was given to Mr. B. E. Brewster, for the purpose of chemical 

 analysis, who reported the following: 



Silica 69.60 



Alumina 17.21 



Ferric oxide 3.66 



Lime 0.74 



Magnesia 0.71 



Soda trace 



Potassa 4.50 



Ignition 3.20 



99.62 



Carbonate of lime is not present in this rock. The percentage of lime 

 is also quite low. The large amount of potassa, associated with only a trace 

 of soda, is of interest in connection with the analyses of other rocks of the 

 West Humboldt Range. On the same ridge occurs a broad band of chert, 

 which, from its withstanding atmospheric agencies better than the surround- 

 ing rock, occupies a prominent position. It has a light-drab color, a con- 

 choidal, splintery fracture, and, under the microscope, presents a micro-crys- 

 talline texture, with minute grains of quartz disseminated through the mass. 

 Mr. Brewster has also analyzed this chert, with the following result: 



Silica 85.77 



Alumina 7.60 



Ferric oxide 0.89 



Lime ! 0.27 



