248 University of California Publications in Geology [Vol. ll 



they are usually smooth and free from large rock masses. In order 

 to get good exposures of well bedded cherts it is usually necessary to 

 seek artificial cuts or sea-cliffs, or look in the trenches of streams 

 which are rapidly cutting downward. 



Massive chert may occur as lenses within a formation of well bed- 

 ded chert, so that one gets prominent outcrops of the former in areas 

 over which there are otherwise no outcrops, but in which the presence 

 of well bedded chert is indicated by the angular fragments in the soil. 



CONTORTION OF CHEETS 

 The well bedded varieties of chert, and occasionally the more 

 massive varieties, are often greatly disturbed, showing intricate plica- 

 tions. Since the associated sandstones are not thus folded it would 

 appear that during the deformation of the Franciscan group there 

 had been a considerable localization of movement in the chert forma- 

 tions. The presence of many bands of shale makes the chert incom- 

 petent and susceptible to folding. Plate 25a shows the character of 

 this deformation. 



BRECCIATION OF CHEETS 



Some of the cherts are traversed by numbers of fissures, so that 

 they break into a great number of little splinters. Others are tra- 

 versed in all directions by lines of slight discoloration, and it appears 

 probable that these lines represent former fissures in the chert which 

 have been recemented. 



In some places, the deformation of the chert has been more rapid 

 or more intense, so that large masses of chert have been brecciated 

 and the bedding entirely obliterated. Often these chert breccias are 

 recemented so that the rock is nearly as strong as an ordinary chert. 

 Near faults, the brecciation has, in some cases, been so intense that 

 the component particles are too fine to be seen with the eye, giving 

 a very fine grained rock with an irregular granular fracture. In 

 some places the brecciated chert is white, earthy in appearance, and 

 crumbles under the hammer instead of breaking into blocks. 



MINOE IEEEGULAEITIES IN BEDDING 

 When one looks at a section of well bedded, red chert, he gets the 

 impression that the individual beds of chert and shale are persistent. 

 In many instances they are so for limited distances, and it is some- 

 times possible to trace a single bed of chert, in an outcrop, for as much 

 as thirty feet. However, when chert beds are examined in detail, it 



