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University of California Publications in Geology [Vol.11 



present, and the amount of iron oxide which they contain. In texture 

 they range from loose porous varieties which will adhere to the 

 tong'ue, up to dense compact varieties which look like wood opal. The 

 bedded cherts of the Monterey group break with rather jagged sur- 

 faces. They seldom show the smooth conchoidal fracture of the 

 Franciscan cherts, probably due to some difference in the condition 

 of the silica which composes them. They show a dull earthy luster 

 and not the waxy luster characteristic of the Franciscan cherts, and 

 of the flinty cherts of the Monterey. 



One very characteristic feature of the bedded cherts of the Mon- 

 terey is the minute lamination. Each bed of chert shows a great 

 number of fine bands of different color, or different shades of the same 

 color, parallel to the bedding of the chert. These bands are so thin 

 that there may be twenty or thirty of them in a layer of chert an inch 



Fig. 11. Discordance of lamination within a single bed of chert. 



in thickness. This lamination of the chert beds is almost universal, 

 but some bedded white cherts fail to show it and occasionally a black 

 bedded chert is not laminated. 



In spite of the fact that this lamination is so well developed and 

 is nearly parallel to the bedding, the cherts show very little tendency 

 to cleave along the laminae. Occasionally there may be a slight 

 tendency in this direction, but it is very exceptional, and as a general 

 rule, the chert beds break up into fragments of irregular form along 

 cracks which run transversely to the bedding planes. 



As revealed in cross sections of the chert bed, the laminae are indi- 

 cated by bands of different color. These often run nearly parallel 

 with the bedding of the chert and are remarkably regular, though 

 there are numerous exceptions to this statement, some of which are 

 rather peculiar. 



In certain specimens a sort of false bedding is shown by the 

 laminae of the bed. This relation might be explained on the supposi- 

 tion that there had been a certain amount of scouring by the currents 

 which deposited the material of the chert (fig. 11 and plate 33b). 



