258 University of California Publications in Geology [Vol. ll 



be the most favorable on account of the greater transparency of the 

 sections, but it was soon found that these highly siliceous cherts sel- 

 dom show well preserved radiolaria. In thin sections of such rocks, 

 the radiolaria are usually represented only by vaguely defined, rudely 

 circular or elliptical areas a little clearer of inclusions than the rest 

 of the section. Apparently, in cherts low in iron oxide and earthy 

 material, when the amorphous silica began to crystallize there was 

 little to preserve the outline of the original form, and there was a 

 tendency for the silica of the matrix to merge with the silica of the 

 radiolarian skeleton so that the sharpness of the original outline be- 

 came lost. In the earthy variety there was not this tendency to 

 fusion. The earthy material, besides its effect in preventing fusion 

 of silica of the matrix with that of the skeleton, makes the radiolaria 

 more readily visible because of the contrast between the clear areas 

 and the dark base. 



These shaly cherts differ from the siliceous red cherts in that it is 

 possible to dissolve out a large part of their iron oxide with hydro- 

 chloric acid. A thin section of a typical siliceous red chert was 

 boiled in 1:1 acid for several hours without producing any notice- 

 able effect upon it. A thin section of a shaly red chert from Red Rock 

 Island, rich in iron oxide, was treated with acid and most of the iron 

 oxide dissolved. It was then apparent that the matrix was composed 

 of a cryptocrystalline aggregate that showed a low birefringence and 

 appeared to be chalcedony. The chert contained a few well preserved 

 radiolaria, but after the acid treatment they became almost invisible. 



One shaly chert, from Point Richmond, is peculiar in that it con- 

 tains large amounts of a mineral that appears to be glauconite. This 

 mineral ranges from a deep green to a pale yellowish green, and is 

 non-pleochroic. It shows no definite structure or cleavage and occurs 

 in masses of irregular form. The relief of the mineral is high and, 

 on testing grains in suitable oils, it is found that the index of refrac- 

 tion is a little above 1.60. Between crossed nicols, it is seen to be a 

 very fine grained aggregate. On rotating the stage, irregular wavy 

 areas of darkness pass over the mineral, but the component grains 

 cannot be made out with the highest powers. The birefringence ranges 

 from almost zero to a little over .020 ; the variable values appear to be 

 due to interference between overlapping scales or flakes of the min- 

 eral. In the shaly part of the rock some material is found that, from 

 its optical properties, appears to be kaolinite. Here and there are 

 seen some very fine irregular flakes of quartz. 



