594 EDWARD SAMPSON 



at an early period, probably immediately after eruption. They 

 believe that "The igneous rocks as they cooled down exhaled vapors 

 or solutions of magmatic origin, rich in dissolved silicates of soda 

 and other bases. These were the agencies which albitized and de- 

 composed the lava, and any excess must have escaped into the sea 

 water. "^ With these pillow lavas are associated cherts which carry 

 radiolaria and the authors believe that the cherts were formed by 

 the accumulation of their remains, pointing out that such siliceous 

 waters would be particularly favorable to their growth. 



Lawson has proposed^ still another source for the siliceous waters 

 from which he beheves the Franciscan cherts were deposited. 

 These cherts are not so intimately associated with volcanic rocks 

 as are the cherts of many other regions, but pillow lavas are present 

 in the Franciscan group. However, the evidence that these cherts 

 have been in a gelatinous state is unusually clear, thin sections 

 showing much isotropic silica and spherules of chalcedony. More- 

 over the field relations of these rocks are peculiar. "Most of the 

 individual occurrences .... are of very Hmited extent, occupying 

 only a few acres, or only a fraction of an acre, and it seems impossible 

 to conceive that they had any other than a very local origin. "^ 

 Where thick beds are found the individual members are seen to be 

 but lenses of very limited extent. The whole aspect of the occur- 

 rences is such as to suggest a very local origin, and Lawson has 

 proposed the theory that they are deposits from siliceous springs. 



We may conclude, concerning the cherts of Notre Dame Bay, 

 that their intimate association with volcanic rocks, as is so generally 

 the case, indicates a genetic connection. The writer believes 

 that they are chemical precipitates. The bedding of the sedi- 

 mentary cherts is so uniform that they cannot have had such a very 

 local origin as the Franciscan cherts. The widespread pillow lavas, 

 usually forming part of chert-bearing sections, are considered of sub- 

 marine origin. The tuffs associated with the cherts show no evidence 

 of shallow water origin, such as ripple marks or cross bedding; 



^ Ibid., p. 245. 



2 A. C. Lawson, " Sketch of the Geology of the San Francisco Peninsula," iSth 

 Ann. Kept., U.S. Geol. Survey, pp. 399-476. 



3 E. F. Davis, op. cit., p. 245. 



