1918] Davis: The Radiolarian Cherts of the Franciscan Group 285 



In some of the beds, the laminae are seen to abut upon the bound- 

 ing surface of the bed at a notable angle (fig. 12). This is also shown 

 in plate 34a. This relation also might be explained as due to scour- 

 ing of the currents which deposited the coarser sediments. 



Occasionally a layer of chert will show a lenslike termination. In 

 such instances the laminae of the chert may follow the walls of the 



Fig. 12. Discordance between lamination and bounding surface. 



Pig. 13. Termination of a eliert layer showing lamination in closed curves. 



Fig. 14. Peculiar lamination in Monterey chert. 



chert layer, and close upon themselves as indicated in the diagram 

 (fig. 13). Obviously a relation of this kind cannot be explained by 

 current scouring in the deposition of later material. 



Occasionally, in the center of a thick bed of chert, the laminae 

 will have the structure indicated in figure 14. The outer bands run 

 through in a general way parallel to the surface of the chert bed, but 

 the laminae in the interior close upon themselves. If the laminae 

 were due to variations of thickness in original deposition, one would 

 not expect them to have the relations shown in the figure. 



