1918] Davis: The Badiolarian Cherts of the Franciscan Group 267 



OCCUEEENCE OF THICK BEDS OF SHALE 



Most of the shales are seen in thin partings between beds of chert. 

 However, there are occasional thicker beds of shale associated with 

 the cherts. Some of these have already been mentioned, occurring at 

 contacts between sandstones and cherts. In addition to these there 

 are occasional zones in the midst of otherwise regularly and thinly 

 bedded chert, which consist entirely of red shale. These may have 

 thicknesses from a foot or so up to three, four or five feet, and occa- 

 sionally even more. Such extra thick layers of shale may be fairly 

 common. They only appear in artificial cuts, and it is probable 

 that they are not long exposed there on account of their rapid dis- 

 integration. 



On Mount Diablo there is one notable occurrence of this sort, in 

 which the shale has a thickness of twenty feet. This shale is exactly 

 like that which is ordinarily found as a parting between the beds of 

 chert. In this thick bed there are a few thin ribs of red chert. The 

 total amount of chert is small, aggregating about six inches out of the 

 whole twenty feet. 



Near the base of the Ingleside chert in the neighborhood of Twin 

 Peaks there is a bed of red shale several feet in thickness. It is with- 

 out bedding planes or banding of any sort, but is traversed by num- 

 bers of irregular cracks so that it breaks up into a large number of 

 small irregular fragments. It contains numerous inclusions of some 

 soft white material which looks like a greatly altered volcanic ash, 

 although it is impossible to get any sort of a thin section of this 

 material in which it might be examined microscopically. 



Besides these small fragments there are rounded masses from one- 

 half inch to an inch in diameter, which may be lapilli. These are 

 roughly spherical in form and are coated on the outside with a layer 

 of manganese oxide. The central cores of these bodies are sometimes 

 unaltered. In such cases there is a dark brown to black core sur- 

 rounded by soft white material apparently the same as that which 

 composes the small white fragments found in the same shale. This 

 white material is often discolored, and in such cases the lines of dis- 

 coloration are concentric with the central core. A thin section of one 

 of these cores shows laths of feldspar embedded in a glassy matrix 

 which is sprinkled with minute particles of magnetite. The feldspar 

 crystals are bent and splintered. They show a tendency to radiate 

 from centers, four or five crystals being sometimes seen in an inter- 

 secting cluster. 



