264 University of California Publications in Geology [Vol. 11 



than the matrix in which they are found. The material of the con- 

 cretion consists of a ground mass of fine cryptocrystalline silica stained 

 with streaks and grains of red iron oxide. Embedded in this ground 

 mass are a number of radiolaria. One thin section across the boundary 

 of a concretion showed a sharp line of contact between the concretion 

 and the iron-rich chert in which it rested. 



In a slide from one of these concretions considerable spherulitic 

 chalcedony was found. Spherulitic chalcedony, as will be shown later, 

 is often found in cherts which have been metamorphosed by igneous 

 action. The three localities where these concretions are found are 

 near intrusive igneous rocks and there is a possibility that the con- 

 cretions may have been produced in some way as a result of meta- 

 morphism. However, they do not occur immediately at the contacts. 

 They appear to be original features of the cherts; this is confirmed 

 by the occurrence in them of radiolaria and by the general similarity 

 of their material to the matrix in which they occur. The deflection 

 of manganiferous bands around them may be taken to mean that the 

 concretion was produced at a time when the chert was still plastic. 

 A further indication of their original character is found in the fact 

 that they are often surrounded by shale coatings. Under the influ- 

 ence of metamorphism the shale in the cherts usually disappears. 



Shale Partings of the Radiolarian Cherts 

 In the region of San Francisco Bay and in all other parts of the 

 Coast Ranges seen by the writer, the chert beds are separated by shale 

 partings. In Oregon, in the Port Orford quadrangle, 14 the chert is 

 occasionally interbedded with thin layers of volcanic ash, instead of 

 the usual shale. 



BED SHALES 



The most common variety of shale is red in color and occurs with 

 the red cherts. It shows no lamination or fissility and breaks into 

 irregular pieces. As a rule, it is a little harder and more coherent 

 than an ordinary clay shale, but some exceptional varieties are ex- 

 tremely soft and break up easily. The rock is remarkably fine grained 

 and shows a somewhat irregular fracture similar to that of a broken 

 cake of chocolate. The typical red shale closely resembles chocolate in 

 manner of fracture and in texture. In most instances it is impossible, 



14 Diller, J. S., Port Orford Folio, U. S. Geol. Surv. Folio no. 89, p. 2, 1903. 



