1918] Davis: The Radiolarian Cherts of the Franciscan Group 259 



There is no sharp distinction between the siliceous red cherts and 

 the shaly red cherts. In fact, considering the red varieties alone, it 

 would be possible, in a large number of specimens collected from 

 various places, to arrange a gradational series from the most siliceous 

 red chert down to the softest of the red shales. All varieties are 

 found and some of the hand specimens, if seen alone, could with diffi- 

 culty be classified as cherts or shales. "While all intermediate varie- 

 ties between two distinct types may be found, it is possible at each 

 place to make clear distinctions between the shales and cherts. In 

 the field one can always make the distinction on the basis of hardness. 

 No matter how soft the chert, the shale partings are always softer 

 so that there is a marked distinction between adjacent beds. Chert 

 beds are never observed to pass into shale beds when traced along 

 the strike. A further distinction is based on the presence of radio- 

 laria. The harder layers of chert always contain numerous radiolaria. 

 The softer shale partings never contain more than one or two in each 

 thin section, and usually do not contain any. 



The red cherts seldom show any fine lamination, such as will be 

 described later in connection with some of the lighter colored varieties. 

 The rhythmic bedding is much better developed in these red cherts 

 and shales, but fine lamination in individual beds is almost unknown. 



VEINING OF THE CHEKTS 



One of the characteristic features of the Franciscan cherts is the 

 large number of veins which cut them. Generally they consist of white 

 quartz, that contrasts strongly with the deep colors of the cherts. 

 Besides quartz, chalcedony or manganese oxide may form veins in 

 cherts, and iron and manganese oxides may coat the walls of open 

 fissures. This characteristic veining is not confined to the red cherts, 

 but may occur in any of the well bedded varieties. 



Each bed of chert usually contains its own system of veins ; the 

 fissures seem to have been confined to the more brittle cherts and did 

 not extend into the shales (plate 26b). Where the shales have been 

 hardened by heat, some veins may run through both the shales and 

 cherts. Even in these there is a strong tendency for veins to be lim- 

 ited to chert. The planes of these veins are, with few exceptions, 

 approximately normal to the bedding of the cherts. They vary in size 

 from those which can scarcely be seen with the microscope in thin 

 sections, up to veins a quarter of an inch in width. Usually they are 

 less than a sixteenth of an inch wide. As described later, wider veins 



