1918] Davis: The Radiolarian Cherts of the Franciscan Group 253 



They stand out on exposed surfaces. The chert in which they are 

 found appears to be of the normal red type, containing numerous 

 radiolaria. The form of the concretions is ellipsoidal or irregular, 

 showing subangular boundaries with rounded corners. Some occur 

 in the form of elongated masses, which may be regular or extremely 

 irregular. The photographs on plate 29 show a few of these con- 

 cretions. Plate 29a is a fractured surface on which some of the 

 concretions are revealed in section. 



Occasionally concretions are found in chert which is banded with 

 manganese oxide. In this case the bands may show a deflection 

 around the periphery of the concretions, or the bands may abut upon 

 the borders of the concretion. 



The concretions in North Berkeley are often separated from the 

 matrix which contains them by a coating of soft red shale, identical 

 with the red shale which ordinarily forms the partings between the 

 beds of chert. Many concretions at this locality, however, do not 

 show this rim of red shale. In the occurrences in the Santa Ynez 

 Mountains, red shale was not noticed surrounding the concretions. 

 There, a thin layer of manganese oxide may surround the concretion. 

 In many cases, there is no coating of different material around the 

 concretion, but it is differentiated from the matrix by color or physical 

 nature of its substance. 



The material of the concretions is found to be much like that of 

 ordinary radiolarian cherts. The principal difference between the 

 concretion and the matrix is usually in color. The color of the con- 

 cretion may be red, yellowish, pink or white. Often it is pale pink 

 in color, showing irregular streaks of red or white material. One or 

 two instances are found where a single band of different color appears 

 within the concretion nearly concentric with its center. Usually 

 there is not the slightest indication of regularity in these streakings. 

 Some concretions consist of material nearly white in color and some- 

 what more granular than ordinary chert, and containing scattered 

 grains and streaks of manganese oxide. 



Petrographic Features op the Radiolarian Cherts 



Chert, according to Van Hise, 11 is a general term used to include 

 all forms of finely crystalline, non-fragmental silica, including opal- 

 ine, semicrystalline and completely crystalline varieties. It is dis- 



ii A Treatise on Metamorphisrn, IT. S. Geol. Surv. Monograph 47, 1904. 



