V 



35- University of California. [Vol. 2. 



Ranges, yet in this limited section there are not less than 31 geo- 

 logical units, many of them complex within themselves, which 

 have been found worthy of cartographic representation upon the 

 scale adopted, viz., 1 : 12,000. The chief results of the study are a 

 partial elucidation of the later Tertiary history of the region of the 

 middle Coast Ranges. The subdivision of the older series of for- 

 mations, such as the Franciscan, Shasta-Chico, and Monterey, has 

 not been attempted on the map, although certain possibilities in 

 this direction are indicated in the text. Other portions of the 

 middle Coast Ranges, in the same geological province as that with 

 which we have here to deal, are known to afford better opportuni- 

 ties for the study of these older Mesozoic and Tertiary formations, 

 and all that could be done in the way of cartographic subdivision, 

 even on the advantageous scale adopted, would have contributed 

 little to our knowledge of their stratigraphic composition and 

 internal relations. 



In contrast to the broad mapping of these older series, the 

 various formations of Pliocene age are rather minutely, though not 

 exhaustively, subdivided; and in the relations of these minor sub- 

 divisions we have been able to decipher a new and most interesting 

 chapter of Coast Range geology. 



The work has engaged the attention of the senior author in a 

 rather desultory way for several years past, while his collaborator 

 contributed much to the geological mapping, and studied par- 

 ticularly the petrography of the volcanic rocks when he was a 

 graduate student at the University of California. The senior author 

 assumes responsibility for the general discussion, while the section 

 dealing with the microscopic petrography of the volcanic rocks is 

 the work of Dr. Palache. 



The topographic basis of the geological map is the work of the 

 late W. H. Otis, and is for the most part a very faithful representa- 

 tion of the relief, although there are portions of it which fail to 

 express the surface features adequately to the scale upon which it 

 is constructed. 



The authors are under obligations to Professor J. C. Merriam 

 for the determination of the various fossils referred to in the text, 

 except where otherwise stated. 



