1918] Davis: The Radiolarian Cherts of the Franciscan Group 289 



point of difference from the bedded cherts, which contain isotropic 

 silica. 



The lamination observable in some varieties runs in general parallel 

 to the bedding. It may not appear at all in hand specimens, although 

 in thin section the laminae are revealed in the form of alternating 

 bands containing different proportions of organic material. Some 

 specimens show minute streaks of calcareous matter which effervesces 

 with acid. In thin sections these bands are seen to be composed of 

 granular masses of calcite or of strings of calcite granules embedded 

 in the chalcedonic matrix of the rock, running parallel to the bedding. 



In other varieties the black flinty chert shows a very pronounced 

 alternation with layers of compact, white, earthy silica. The alter- 

 nate layers range in thickness from one-eighth to one-quarter of an 

 inch, the contrast in color giving this rock a very striking appearance 

 (plate 34b). 



Sometimes the banding is due to the alternation of black flinty 

 chert with very thin stripes of white chalcedonic chert. In such cases 

 the only difference between material of the stripes is the presence or 

 absence of organic matter. 



The lamination in these flinty cherts is often rather irregularly 

 developed. Sometimes the laminae show a remarkably wavy or crum- 

 pled appearance. There may be a lenslike development of the differ- 

 ent laminae. In other cases white bands run for a distance and die 

 away to reappear again an inch or so further along. In such in- 

 stances if one examines the intervening space carefully, he may see 

 that, while the white color is not developed, the band is marked out 

 by slightly different character of material, or by the presence of sev- 

 eral exceedingly minute white lines. It would appear as though the 

 bands were due to silica of different structure in which locally the 

 white color had not been developed. In some specimens the white 

 bands appear on weathered surfaces, but not on fresh fractures. 



This type of chert, though not found rhythmically interbedded 

 with terrigenous shales, is often interlaminated with white siliceous 

 shales in which it may occur as lenslike beds, short lentils or nodules. 

 Small irregular streaks of this black flinty chert are frecpiently found 

 in the midst of the layers of the ordinary bedded cherts, as before 

 stated. Sometimes thick masses of the gray or white earthy cherts, 

 without shale partings, show a more or less regular alternation with 

 bands of black flinty chert, so that the general appearance is similar 

 to that seen where ordinary chert is interbedded with shale part- 

 ings. In all cases where they are encountered, the flinty cherts show 



