1918] Davis: The Radiolarian Cherts of the Franciscan Group 249 



is found that they are not so continuous as they seem at first sight. 

 The bedding is often interrupted by numbers of cross faults of small 

 throw and it shows a variety of other irregularities. Since it is be- 

 lieved that these irregularities cast light on the question of the genesis 

 of the chert, they will be described in some detail. 



Tig. 1. Beginning of a lens of massive chert. 



At various places throughout well bedded cherts there are lenticu- 

 lar masses of chert of considerably greater thickness than the adjacent 

 beds, ranging from one to eight feet in thickness and in length from 

 five to thirty feet. They are usually lighter in color than the enclos- 

 ing thin bedded cherts. Figure 1 indicates the beginning of a lens of 



Fig. 2. Pinching and swelling of beds of chert. 



Fig. 3. Pinching and swelling of beds of chert, showing manner in which 

 irregularities in thickness are compensated. 



this sort, showing the way in which the surrounding beds accommo- 

 date themselves to it. 



A very common feature is the pinching and swelling of chert beds ; 

 it may be seen in almost any section of the chert and is illustrated in 

 figures 2 and 3 and plates 25b, 27a, b, and 28a. The usual range of 

 pinching and swelling is indicated by the illustrations. 



