1918] Davis: The Badiolarian Cherts of the Franciscan Group 287 



specimen appears of uniform color, may show in thin section a 

 streaky appearance which resembles somewhat the flow lines in felsitic 

 igneous rocks. Cherts which do not show well developed lamination, 

 still show this streaky appearance. 



Sometimes the moulds of foraminiferal shells have been filled by 

 chalcedony, but in the greater number of the bedded cherts studied, 

 the material of the shells has been leached out, leaving the empty 

 mould of the shell ; or else the original lime carbonate is still present, 

 and the chambers are empty or filled with calcite. In those cases where 

 the shell is replaced by silica, the silica shows a different degree of 

 crystallization from that in the matrix of the rock. If the matrix is 

 amorphous, there is notable difference because the silica filling the 

 cavity is generally chalcedonic. If the matrix is chalcedonic there 

 are differences in grain. 



Aside from a few foraminifera there is a notable scarcity of 

 organic remains in the chert. Sometimes structures remotely resem- 

 bling diatoms are made out, but as a rule there is nothing which 

 would be referred to as organic in its origin. No radiolaria were 

 found in any of the sections of chert which were studied. 



The shale partings which separate the layers of the bedded cherts 

 of the Monterey group are usually ordinary terrigenous shales. Occa- 

 sionally yellow or brown bituminous shales may separate the chert 

 layers, but such instances are not so common. No instances are known 

 where black bituminous shales come between cherty layers. 



In the Telegraph Canon Quarry, east of Berkeley, a bed of 

 medium, coarse grained sandstone two inches thick is found lying 

 between two chert layers in the place of the fine grained shale which 

 ordinarily occurs. Above and below this layer the chert is separated 

 by shale partings. In hand specimen, this sandstone is well cemented 

 and of dark yellow to brown color. Under the microscope it is found 

 to consist of angular fragments of quartz and feldspar cemented by 

 iron oxide. The feldspar is largely plagioclase and is so little altered 

 that in ordinary light the grains of feldspar look much like the quartz 

 grains, being absolutely clear and limpid. No well rounded frag- 

 ments of any sort are seen, all the grains having sharp corners. 



This is the only case known to the writer where a thin sandstone 

 parting occurs between two beds of chert. However, beds of a peculiar 

 sandstone to be described later are often interbedded with cherts and 

 shales. This interbedding may be rather regular so that beds of sand- 

 stone a few feet thick alternate with horizons of bedded chert and 

 shale which are only a few feet thick. 



