EXPLANATION OF PLATE 28 



Fig. A. Franciscan chert, Tennessee Cove, Marin County. Shows pinching 

 and swelling of chert, lenslike beds of chert, and the wedging out of shale 

 partings. 



Fig. B. Franciscan chert, Twin Peaks, San Francisco. Shows the occur- 

 rence of the typical red radiolarian chert in short lenses and nodules. 



[416] 



