THE ORIGIN OF CERTAIN SILICEOUS ROCKS 37 1 



3. Chemical precipitates. 



4. Igneous deposits. 



5. Replacements of clays. 



6. Replacements of limestones. 



7. Replacements of dolomites. 



The most comprehensive discussions of this subject are 

 those of Griswold and of Rutley. But these writers do not 

 agree in regard to the origin of the rocks, the former believing 

 them to be fine sediments, the latter believing them to be 

 replacements of dolomites or limestones. 



My own opinions have not been based upon such micro- 

 scopic study as either Griswold or Rutley devoted to this work, 

 and for that reason were not worthy of special consideration. 

 Since I began to study the geology of the coast ranges of 

 California, however, I have, I believe, seen much that throws 

 light upon the origin of the novaculites and similar rocks. 



These observations embrace those upon geological structure, 

 gross and microscopic structure, composition and geological 

 relations, and lead me to believe that the white siliceous shales 

 so characteristic of the Tertiary, the jaspers and the diatomaceous 

 beds are only various phases of the same thing. 



I am glad to say that Dr. H. W. Fairbanks, whose acquaint- 

 ance with the coast range geology makes his opinion especially 

 valuable, expresses similar views on this subject. Dr. Fair- 

 banks, however, has undertaken a thorough study of this subject 

 from which important results may be expected. 



J. C. Branner. 



Stanford University, California. 

 June 4, 1898. 



