1918] Davis: The Radiolarian Cherts of the Franciscan Group 317 



In thin section, the cherts are seen to contain radiolaria. Some 

 of the shale partings contain so many radiolaria that they have a 

 somewhat gritty texture and were on this account described by earlier 

 workers as fine grits. 



In a few instances beds of dark limestone containing radiolaria 

 and sponge spicules alternate with the siliceous beds. As a general 

 rule, however, the cherts contain no lime. Even those cherts which 

 contain casts of calcareous organisms, are free from lime, all traces 

 of it having been leached out. 



Under the microscope, the cherts show a fine grained siliceous 

 matrix in which the tests of the radiolaria are embedded. In some 

 instances the siliceous ground mass is clear ; in others it is dark and 

 turbid from the presence of fine particles of carbonaceous material 

 or ferrous minerals. Sometimes minute prisms of rutile and zircon 

 are seen. Between crossed nicols the matrix shows the characteristic 

 speckled appearance of cryptocrystalline silica, similar to that shown 

 by flint. 



The chert beds are laminated on a very fine scale. This structure 

 is best seen in the light gray cherts, where it consists of alternating 

 bands of lighter and darker material. In thin section the lamination 

 is seen to be due to regular layers of dark amorphous material and 

 finely granular mineral particles. 



The radiolarian tests vary greatly in their abundance. In some 

 beds they are thickly crowded together, while in others they are 

 sparsely scattered through the siliceous matrix. The silica within 

 the radiolarian tests is generally clear, and nearly transparent, being 

 free from the rutile needles, and dark substances disseminated in the 

 ground mass. It is either granular or, more commonly, it has a radi- 

 ate fibrous structure. The preservation of the tests varies consider- 

 ably. In some of the cherts the boundaries between the siliceous cast 

 and the siliceous matrix of the rock are vague and the radiolaria may 

 not be easily visible if the ground mass is clear. Between crossed 

 nicols the radiolarian tests appear as circular areas of speckled or 

 bright light on a nearly dark ground. In the shaly cherts the radio- 

 laria are much better preserved. 



In many of the cherts, sponge spicules are noticed in close associa- 

 tion with the radiolaria, and some contain minute cubes of pyrite. 



In most of the sections, detrital material appears to be absent. 

 An exception to this is found in the presence of very minute flakes 

 of mica. In some of the dark, hard platy varieties there is a certain 



