288 University of California Publications in Geology [Vol.11 



In one instance a two foot bed of this sandstone was found bedded 

 with cherts and shales and separated from the chert on either side 

 by a shale parting-. Within the mass of the sandstone there were 

 several lenses of chert from six inches to two feet long, lithologically 

 like the cherts above and below. These lenses were at different hori- 

 zons in the sandstone bed. Some were in contact with the shale part- 

 ing at the base of the sandstone, while others were entirely included 

 within the sandstone. 



Flinty Cherts. — The flinty cherts of the Monterey group differ 

 from the bedded cherts in several ways. They are usually in thicker 

 bodies, not so well bedded, and they do not show a rhythmic alterna- 

 tion with ordinary shales. They seldom exhibit the lamination shown 

 by the bedded cherts and when they do show lamination it is not so 

 evenly developed. They are generally black in color, though occa- 

 sionally they are dark brown with a red or yellow cast. They usually 

 show a fair conchoidal fracture, though the edges which may be thus 

 produced are not as sharp as those which may be produced in ordinary 

 chalcedony. They possess a vitreous or waxy luster, while the bedded 

 cherts are usually rather dull or earthy. 



The black color appears to be due to the presence of small amounts 

 of bituminous matter. In thin section the silica is seen to be stained 

 with dark brown material. On breaking a piece of this rock under 

 the hammer it gives rise to a fetid odor strong enough to be perceptible 

 for a distance of several feet away. On heating, these cherts gradu- 

 ally lose their color and become gray. 



The black flinty cherts contain, scattered through their mass, 

 large numbers of perfectly preserved foraminiferal shells. Under 

 the microscope these are seen to retain their original structures and 

 frequently consist of the original aragonite. In some cases the cham- 

 bers are filled with calcite, and in others they are still open. Occasion- 

 ally only the mould of the shell remains. 



In thin section these cherts are found to consist largely of crypto- 

 crystalline chalcedonic silica stained with organic matter. Between 

 crossed nieols they present a mottled, granular appearance resem- 

 bling greatly the appearance of thin sections of ordinary chalk flints. 

 The larger grains in this aggregate may be seen, on rotating the stage, 

 to possess a fibrous wavy extinction. No isotropic silica is distinguish- 

 able, though there may be considerable present between the minute 

 grains which polarize the light. Its presence is shown on treatment 

 with caustic potash. The presence of much chalcedonic silica is a 



