408 University of California Publications in Geology [Vol. 11 



The Monterey cherts resemble the Franciscan cherts in many 

 ways, as previously pointed out. The latter have here been interpreted 

 as due to siliceous springs associated with the intrusion and extrusion 

 of certain igneous rocks of a peculiar type. The question arises as to 

 whether the Monterey cherts have an origin similar to that postulated 

 for the Franciscan cherts. 



The Monterey cherts appear to have been laid down in the form 

 of gelatinous silica. Evidence has been presented to show that they 

 were not derived from diatomaceous earths. No sources of silica 

 appear possible for the Monterey cherts which were not discussed 

 in connection with the Franciscan cherts. 



During Monterey time there was considerable igneous activity, 

 both extrusive and intrusive. There are many important beds of 

 tuff associated with the Monterey group, and some of the rocks re- 

 ferred to as sandstones contain so much volcanic glass that it is 

 possible that they may represent tuffs, instead of ordinary elastics. 

 Further than this, many igneous rocks are known in the Monterey 

 group which are rich in soda minerals. These show the peculiar altera- 

 tion of feldspar and its replacement by such soda-rich minerals as 

 analcite and natrolite, both of which were cited by Dewey and Flett 

 in support of their idea that the spilitic rocks gave rise to emanations 

 of sodium silicate solutions. Rocks of this type, in association with 

 the Monterey, have been found by Fairbanks, 172 who described an 

 analcite diabase from southern California. 



Haehl and Arnold 173 described basic rocks of somewhat similar 

 nature from the Santa Cruz Mountains in San Mateo County. 



It seems probable, therefore, that the Monterey cherts had an 

 origin similar to that postulated for the Franciscan cherts. 



1" The Geology of Point Sal, Univ. Calif. Publ., Bull. Dept. Geol., vol. 2, 

 pp. 1-92, 1896. 



i"3 The Miocene Diabase of the Santa Cruz Mountains in San Mateo County, 

 California, Proe. Am. Philos. Soc, vol. 43, p. 16, 1904. 



