252 University of California Publications in Geology [Vol. 11 



the figures, but are frequently rather blunt with rounded ends, as 

 shown in plate 28b. 



The terminations of beds in the manner indicated in the illustra- 

 tions are not limited to the chert layers. The shales often terminate 

 in the same manner, so that two bands of chert come together and 

 unite in a single layer (plates 26a, 27a and b). As a rule, in the 

 Franciscan cherts, it is more common for the cherts to show this 

 wedging-out than for the shales to show it. This is a point of contrast 

 with the Monterey cherts. In the Monterey cherts the terminations 

 of shale are rather common, being found about as often as wedge- 

 like terminations of chert beds. 



Fig. 9. Minor lenses and nodules of chert in a thick bed of shale. 



Some beds of chert show thin, lenslike strips of shaly material 

 included entirely within the mass of the chert, and (plate 25b) occa- 

 sionally the separation between the included shaly layers and the 

 adjacent chert is not sharp but is in the nature of a transition. They 

 differ in this way from the shale partings between beds of chert 

 which, as already stated, are always sharp and clean cut. 



In certain instances one may find very thin ribs of chert in the 

 shale partings between two chert beds of ordinary dimensions. These 

 are not persistent, but die away quickly when traced for a few inches 

 or a few feet. 



The radiolarian cherts show, rarely, a kind of structure which, for 

 want of a better term, will be referred to as ' ' concretionary. ' ' In the 

 Santa Ynez Mountains concretionary cherts of this sort are found at 

 two points, one in Aleso Canon and the other at Red Rock on the San 

 Marcos ranch. Structures of this type are also found in the outcrops 

 of chert on Arch Street in North Berkeley. Here and in Aleso Canon 

 the concretions occur in red chert which is rather massive and does 

 not show well developed shale partings. At Red Rock the concretions 

 are found in well bedded red chert. At all these places the concre- 

 tions are very abundant. 



CONCRETION AEY CHERTS 



