254 University of California Publications in Geology [Vol. ll 



tinguished from quartzite in that it is non-elastic. The term, chert, 

 includes both organic and chemical deposits. The definition would 

 include the true jaspers, which consist of finely crystalline quartz, 

 stained with iron oxide. 



The Franciscan cherts show many variations in lithologic nature. 

 They are composed of several forms of silica — both amorphous and 

 crystalline. They show numerous variations in color and in general 

 appearance. The most common variety is the red to brownish-red 

 chert which is well bedded with shale. Beside these red cherts there 

 are very often found green, gray and white varieties. Yellow, brown, 

 black, purple, blue and clear colorless varieties are also encountered. 



THE BED CHERTS 



Typical Red Cherts. The typical red cherts, which are the most 

 common variety, may be described as deep brownish-red in color, 

 showing a smooth conch oidal fracture and breaking with sharp edges. 

 Occasionally the fracture is irregular or hackly. The luster is dull 

 vitreous, inclining to waxy. The rock is dense and compact in tex- 

 ture and is rather hard, ranging between six and seven of the ordinary 

 scale of hardness. Within the red siliceous matrix there are darker 

 spots of clear silica, barely perceptible to the naked eye, which repre- 

 sent the remains of radiolaria. Rarely these spots are white and 

 stand out very distinctly from the matrix. In any case the spots are 

 better seen when the surface of the rock is moistened. With a hand- 

 lens, the spots, are ordinarily quite distinct and, in a few specimens, it 

 is possible to make out some of the structures of the radiolaria. 



Generally the cherts are thin bedded and this fact in conjunction 

 with the prevalence of jointing, causes them to break down into small 

 fragments. Exceptional specimens from thicker beds are massive 

 and do not break up in this way. Spear-heads, brought from the 

 region of Humboldt Bay in northern California, made by Indians 

 from fragments of red chert, are eight inches long. Occasionally one 

 may find in creek beds, solid blocks of red chert having diameters 

 between six inches and a foot. 



In thin section, the red chert is seen to consist of a very fine 

 grained siliceous matrix, colored by disseminated grains of red iron 

 oxide. In this red, semitranslucent to opaque matrix there are minute 

 areas of clear silica usually circular or elliptical in outline. These 

 clear spaces represent the radiolaria, and when the rock has not been 



