1918] Davis: The Radiolarian Cherts of the Franciscan Group 281 



twenty-five per cent of the volume of rock, and the foraminiferal 

 shales may be nearly as abundant. The grains appear to consist 

 largely of quartz and feldspar. They are angular, often sharply so, 

 consisting of chips and splinters showing a maximum diameter of 

 about a tenth of a millimeter and ranging down to grains almost too 

 small to see under the microscope. The feldspars comprise both 

 plagioclase and orthoclase. The foraminifera are often remarkably 

 well preserved. The walls of the shell stand out clearly and all the 

 minute details of the structure are well preserved. The material of 

 the shell wall is aragonite, which is fibrous with extinction parallel to 

 the fibers, these standing perpendicular to the wall of the shell. The 

 minute perforations through the shell wall are preserved and may be 

 seen clearly. The chambers may be filled with calcite and generally a 

 single crystal fills a chamber. In a few instances the chambers of the 

 shells are filled with opal, which may be dark brown in color, but 

 usually is rather pale. 



In addition to the complete and well preserved shells, there are 

 many broken fragments of foraminifera scattered through the matrix. 

 These have undergone only a mechanical separation, since the minute 

 perforations through the ' shell walls are well preserved, and the 

 aragonite of the shell has not been altered. 



"WHITE SHALES" 



Besides the diatomaceous shales and the diatomaceous earths, 

 there are "white shales" of a peculiar type which reveal no trace of 

 diatoms. These are light colored, usually white, gray, or cream col- 

 ored. They break with an angular fracture with sharp definite edges. 

 They are very light and rather soft, but in texture* are firm enough 

 to resist crushing in the hand and are often found in later stream 

 gravels as pebbles. They are very porous, absorbing water readily, 

 and will stick to the tongue if touched to it for a second. With a 

 hand-lens they are seen to be rather compact, showing no sign of 

 granular structure. They possess a luster and an appearance which, 

 save for the absence of the small tubular passages, reminds one greatly 

 of pumice. In these white shales there are numerous moulds of fora- 

 minifera, showing in many cases the delicate details of the external' 

 form of the shells. In some instances the lime carbonate of the shells 

 is still present, not having been leached out. 



Under the microscope these white shales are found to consist 

 largely of opaline silica in which there are very small amounts of 

 argillaceous matter together with a few minute, angular grains of 



