1918] Davis: The Badiolarian Cherts of the Franciscan Group 407 



required some unusual condition not now present in the basins of 

 deposition. As pointed out by Van Hise and Leith, the ellipsoidal 

 basalts seem to be the abnormal factor in their occurrence. 



The question of the origin of the characteristic rhythmic bedding 

 has been discussed. Of all the ideas suggested, two only appear of 

 any promise. These are the idea of rhythmic precipitation due to 

 periodic supersaturation and the idea of colloidal segregation. As 

 pointed out, the idea of rhythmic precipitation is incapable of ex- 

 plaining all the facts, and it is necessary to appeal to the idea of 

 segregation to explain the bedding of the cherts. Certain experiments 

 are cited which indicate the possibility of such segregation. 



At the time of precipitation of the cherts, there must have been 

 numerous radiolaria thriving in the siliceous waters and their skele- 

 tons were accunmlated in the ooze forming on the sea bottom. No 

 calcareous organisms are found, indicating that the conditions were 

 not favorable for their development. The radiolarian skeletons seem 

 to have undergone a segregation also and to have behaved like the 

 other colloidal silica. This might be expected on the idea of segrega- 

 tion, since the silica of the skeletons is in the colloid form and might 

 be expected to behave in somewhat the same manner as the gelatinous 

 silica. It must be admitted that so far it has not been possible to bring 

 this about in an experiment. 



The exact nature and mode of origin of the shale partings in the 

 Franciscan cherts is left in doubt. It is in appearance not an ordi- 

 nary clay shale. Its properties and its chemical composition are those 

 which might be expected if a red mud became consolidated into a 

 rock. On this interpretation it should be regarded as a terrigenous 

 sediment accumulating in the basin in which silica was being pre- 

 cipitated. There is also the possibility that it might be in part, or 

 wholly, a chemical precipitate resulting from the separation of certain 

 substances brought out in the siliceous springs. It might also be a 

 combination of mechanical, terrigenous sediment with chemical sedi- 

 ment of the type suggested. The shale partings of the Monterey 

 cherts appear unquestionably to be mechanical sediments, and we 

 might, by analogy with them, regard the Franciscan shales as mechan- 

 ical sediments of unusually fine grain. However, in the Lake Superior 

 District, cherts occur interbedded with iron oxide, and this iron oxide 

 has been interpreted as a chemical precipitate. In the absence of fur- 

 ther evidence, therefore, no absolute statement can be made, other 

 than there is nothing about the Franciscan shales which opposes their 

 interpretation as terrigenous muds. 



