1918] Davis: The Radiolarian Cherts of the Franciscan Group 261 



pyrite when this is present — since analyses show a lower content of 

 iron oxide than do the red cherts. It is possible by heating small 

 splinters of the green chert in an oxidizing flame to change the color 

 to red, but it never becomes as intense as it is in the unaltered red 

 chert. 



In some instances, deep colored red cherts are found which contain 

 crystals of pyrite. Occasionally such crystals, showing perfectly de- 

 veloped bright faces of the pyritohedron, occur in the midst of solid, 

 red chert little changed by any action of solution. This pyrite 

 may be syngenetic or it may be an example of metasomatic replacement 

 without volume change. 



EESISTANCE TO CHEMICAL WEATHERING 



The cherts are very resistant to chemical decay. In streams drain- 

 ing areas of Franciscan rocks the larger proportion of the boulders 

 at some distance from the source of supply are chert, and of these 

 a large part are of the red varieties. This is in spite of the fact that 

 cherts may occupy only a limited portion of the area drained by the 

 stream. Along the coast in Marin County, where short streams drain 

 Franciscan areas, the beach pebbles are largely composed of chert, and 

 the sands also are found to be composed of fine grains of chert. 



In spite of their high resistance to chemical action they are some- 

 what attacked. This is seen on examination of fragments of chert 

 which have lain on the surface for some time. On these chert frag- 

 ments the radiolarian remains stand up in slight relief, due to the 

 greater purity of the silica filling these skeletons, which enables them 

 to resist better than the impure matrix. Quartz veins also stand out 

 in slight relief on the surface of blocks long exposed to the action 

 of the weather. 



GRAY AND GREEN CHERTS 



While many well bedded green and gray cherts are the result of 

 the decoloration of cherts originally red, there are some of these cherts 

 which do not seem to be due to this sort of action. These occur inter- 

 bedded with the red cherts and the color difference is sharply marked 

 along the lines of bedding. Sometimes green streaks run parallel to 

 the bedding of the red cherts. These facts indicate original differences 

 at the time of deposition. 



