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University of California Publications in Geology [Vol. ll 



Some beds of chert show relations about as indicated in figure 15. 

 Photographs of this sort of banding were obtained, but the details 

 were too faint to bring out in reproductions. 



Sometimes some of the laminae are discontinuous and consist of a 

 series of isolated lenses of different color, along the same general line. 

 In some instances the lamination appears as irregular wavy bands, 

 while the planes bounding the bed are straight and parallel to one 

 another. 



In thin section the bedded cherts are found to be composed largely 

 of amorphous silica ; only occasionally is there much cryptocrystalline 

 chalcedonic silica. Embedded in this amorphous matrix there are 

 numbers of mineral grains, together with some foraminiferal shells. 



Under the microscope the lamination of the cherts is seen to be 

 due to slight differences in the material composing the bands. The 



Fig. 15. Peculiar lamination in Monterey chert. 



nature of these differences varies in different cherts. Some of the 

 cherts are wholly composed of amorphous silica and the lamination is 

 due to the fact that one part of the chert is stained with iron oxide 

 while the adjoining portions are not so deeply stained. In some of the 

 cherts, the lamination is due to variations in amounts of bituminous 

 matter in the different parts. In some instances all the rock may be 

 isotropic, but one part appears slightly more opaque than the adjacent 

 parts, due perhaps to the presence of a small amount of argillaceous 

 material. In such instances the darker bands seen in the hand speci- 

 men correspond to the more transparent material. Some of the lami- 

 nation is due to alternating strips of amorphous and chalcedonic 

 silica. The latter, in the hand specimens, appears as dark gray to 

 black bands with a w&xy luster contrasting with the lighter colored 

 portions of opaline silica, which show a dull earthy luster. The amor- 

 phous silica appears somewhat incoherent and contains more impuri- 

 ties, while the chalcedonic silica is dense and fairly pure. Banding 

 of this kind is liable to be irregular, with ragged outlines. 



Aside from their banding, another feature of thin sections of these 

 cherts is their peculiar streaked appearance. A lamina which in hand 



