302 University of California Publications in Geology [Vol. 11 



In order to compare the behavior of these flints with the Monterey 

 flinty cherts the writer tried certain experiments upon both rocks. 

 A specimen of flint was treated with acid until all lime carbonate 

 was dissolved away. Portions of the white siliceous rind were exam- 

 ined under the microscope and found to consist of isotropic silica. 

 On putting a piece of the flint with the adherent white crust into 

 caustic potash solution for a short time the white rind went into solu- 

 tion very rapidly, leaving the black flint untouched. 



On exposing a piece of black flint to the action of caustic potash 

 for a time, it was found that the opal in the flint gradually went into 

 solution and after a long time the alteration proceeded so deeply that 

 the flint became covered with a white rind of porous silica. On pow- 

 dering this rind and examining it under the microscope it was found 

 to consist, not of isotropic silica, but of anisotropic silica. A specimen 

 of black flinty chert from the Monterey was treated in the same way 

 and similar effects were produced. 



It would appear from the above observations that the flints, and 

 the black, flinty, Monterey cherts consist of an intimate mixture of 

 two types of silica. One type is amorphous and soluble in caustic 

 potash. The other type is anisotropic and not soluble to any large 

 extent in caustic alkalies. Under the action of surface water, in 

 weathering, the anisotropic silica is removed from the flint or from 

 the flinty chert, leaving the opaline silica behind. If the chert or 

 flint be treated with caustic potash the reverse effect occurs, and opal 

 goes into solution leaving the chalcedonic silica behind in a porous 

 mass. 



The flinty cherts of the Monterey group show numerous examples 

 of this change. The exposed surfaces of black cherts are usually cov- 

 ered with a white coating, showing the change which they are under- 

 going. Fragments of black, flinty cherts become bleached so that the 

 soil over such a chert horizon is filled with white angular fragments 

 of bleached chert. Often the flinty cherts will show alteration to 

 white amorphous material along fissures as indicated in plate 35a. 

 In other cases blocks of chert show a coating of white shale resulting 

 from their alteration. This alteration cuts across original banding 

 in the matter shown in plate 35d, indicating that the association of 

 white shale and black chert is due to secondary action. Sometimes 

 a black chert shows white bands developed on a weathered surface, 

 but on breaking it open, the white bands do not appear on the interior 

 though the appearance is such as to suggest a banded structure 

 (plate 35d). 



