1918] Davis: The Radiolarian Cherts of the Franciscan Group 257 



lar transparent spots can be seen in the iron stained matrix of the 

 rock; these represent the nearly obliterated radiolarians. 



On examining these transparent areas between crossed nicols, 

 they present a strong contrast with the matrix, aside from the fact 

 that they are free from iron oxide. They generally consist of chalce- 

 dony in which the fibrous structure is well developed, showing a nega- 

 tive elongation. In some instances the whole mass is a single spheru- 

 lite of chalcedony in which the fibers all radiate from one common 

 center. In many instances the areas may be divided into two or three 

 definite sectors in each of which the fibers radiate from a single center. 

 On rotating the stage, the characteristic black cross will be seen in 

 each of these sectors. Occasionally the clear area is made up of an 

 aggregate of fine grains and presents a gray mottled appearance 

 between crossed nicols. Possibly this material is fibrous chalcedony 

 cut normal to the fibers. Rarely there are grains of quartz filling 

 the areas of the radiolarian tests. The particles which make up the 

 matrix of the cherts usually have a much finer grain than those found 

 in the radiolarian remains. In cherts in which the radiolaria are not 

 well preserved, it is possible to locate their position between crossed 

 nicols by this difference in the size of the grain. 



The abundance of the radiolarian remains varies considerably. 

 In some specimens they are quite sparsely distributed through the 

 ferruginous matrix. In others, the clear spaces are so numerous that 

 they almost touch, leaving little more than the interstitial spaces for 

 the iron stained matrix (plate 30d). Plates 30a, b and c show the 

 usual proportions of matrix and of radiolarian skeletons. 



Shaly Cherts. — The red, shaly cherts are somewhat different in 

 appearance. They contain a larger proportion of earthy material 

 and iron oxide than the typical cherts. They have a dull luster and 

 break with a rough fracture into irregular fragments. They are soft, 

 as compared with the siliceous red cherts, and can be readily 

 scratched with a knife. Often they are rather porous and absorb 

 moisture readily. Examples of very soft shaly cherts are to be seen 

 on Red Rock Island in the Bay of San Francisco, where they occur 

 interbedded with unusually soft red shales. 



Under the microscope these shaly cherts show a matrix so deeply 

 stained with red iron oxide that it is practically opaque. The radio- 

 laria are similar to those in the siliceous cherts save that they are 

 much better preserved. In looking for material for a study of the 

 radiolaria it was at first thought that the light colored cherts would 



