266 University of California Publications in Geology [Vol. 11 



birefringence, and index of refraction is determined as quartz. There 

 are also occasional splinters of a dark, brownish, isotropic material 

 having an index of refraction between 1.62 and 1.63 which appear to 

 be rather basic volcanic glass. These are especially abundant in one 

 red shale which came from beneath a flow of ellipsoidal basalt. 



Interbedded with the soft shaly cherts previously described there 

 are very soft, red shales. The best examples of these are to be found 

 on Red Rock Island in association with beds of very soft shaly 

 cherts. It was possible to remove all the iron oxide from these shales 

 by treatment with acid. During the process the shale broke down, 

 leaving a soft incoherent residue of pale cream color. On qualitative 

 analysis this was found to consist largely of silica and alumina, with 

 a trace of iron and manganese. It contained no calcium or mag- 

 nesium. Apparently it consisted almost wholly of kaolin. Chemical 

 analysis indicates that these soft red shales are much lower in silica 

 than the ordinary varieties. 



The red shales show evidence of leaching of their iron along chan- 

 nels of circulation, just as the cherts do. "When the iron is removed 

 the shale is either greenish or gray in color; the texture and general 

 appearance are the same as before. 



In some of the red shales peculiar green spots are found, due appar- 

 ently to the local reduction of iron. These spots are circular or ellip- 

 tical in cross section, and almost every one of them shows a tiny dot 

 of yellow in the center, probably limonite. 



GEEEN AND GRAY SHALES 



"When the green and gray shales are examined in thin section they 

 are seen to be much like the red shales from which the iron has been 

 leached by dilute acids. They show a roughly oriented aggregate of 

 the same sort of material which is left in the acid residue of the red 

 shales. The green shales turn pink or faint red, like the green cherts, 

 upon heating in the air. 



CARBONACEOUS SHALES 



At Tennessee Cove in Marin County and in Bagley Canon at Mount 

 Diablo, shale beds of another type are found. They are black shales 

 containing considerable carbon and after ignition, they become grayish 

 white. They are interbanded with white or gray cherts. 



