262 University of California Publications in Geology [Vol. ll 



Another variety of light colored chert, which cannot be shown to 

 be altered red chert, is a translucent rock with a vitreous luster and 

 bright, clear, green color. This chert is very hard, frequently being 

 hard enough to scratch quartz. It has a perfect conchoidal fracture. 

 In thin sections it appears to be nearly colorless, with a slight clouding 

 due to minute inclusions, and between crossed nicols it is seen to be 

 made up of an extremely fine grained aggregate of cryptocrystalline 

 silica. Another hard, vitreous variety like the green chert is black in 

 color, resembling some of the flints from the Chalk. In thin section 

 it also consists of colorless cryptocrystalline silica, with a very small 

 number of minute black specks. Sections of both the black and the 

 green flinty cherts show occasional spots of vague outline, that are 

 clearer than the rest of the rock. The dimensions of these spots are 

 about the same as those of the radiolaria in other cherts, and they 

 appear to represent original radiolarian remains. That this is the 

 correct interpretation seems certain from the fact that in some of 

 these cherts that contain many inclusions the details of the outlines 

 of radiolaria are clearly seen. Rarely a chert of this sort is found 

 which is colorless in hand specimen and shows the faint cloudiness of 

 chalcedonic silica. 



LAMINATED BLACK AND WHITE CHERTS 



This variety of bedded cherts shows well marked lamination, a 

 character not common in Franciscan cherts. They are white to cream 

 colored with fine bands of black chert running through them parallel 

 to the bedding. They are interbedded with gray or dirty yellow shale. 

 Good exposures of this variety are to be found in the vicinity of Red- 

 wood City and on the military road between Fort Baker and Fort 

 Barry in Marin County. 



Moistened surfaces of this chert show distinctly the small, clear, 

 round spots representing radiolarian remains. In thin sections the 

 radiolaria usually do not appear to advantage, on account of the lack 

 of contrast between them and the matrix in which they lie. The black 

 and white banding is due to differences in the purity of the silica in 

 the different bands. The black bands are composed of silica which is 

 purer and clearer than that in the white bands. They appear darker 

 in reflected light because they are more translucent. 



One specimen of this type of chert shows alternating bands of dark 

 translucent and creamy white silica. There are places where veinlets 

 of black chert run off from the black bands and cut across the white 



