Vol. IV] ANDERSON AND MARTIN— NEOCENE RECORD 35 



scribed, but while some of these differences are more or less 

 constant they can not serve as a basis of a division into forma- 

 tional groups, except locally, and with some reserve. 



The areal mapping of the various groups of the Neocene is 

 attended with the same sort of difficulties as the stratigraphic 

 determination, and for the same reasons. 



Paleontology is a useful aid in making, or rather in identify- 

 ing, the divisions, but the final word has not yet been said as 

 to the range of species ; in fact, there are still many undescribed 

 species to be found in different portions of the series. Among 

 the important facts to be considered in connection with the 

 faunas as reflecting the changes in physical geography is the 

 alternation of sub-tropical and boreal faunas, in the principal 

 divisions of the series, the Diatomacese being of boreal aspect. 



THE SAN JUAN DISTRICT 

 General Statement 



Location. — The territory comprised in this district includes 

 a relatively small area in the north-central part of San Luis 

 Obispo County extending from the Carrizo plain northwest- 

 ward to the Salinas Valley, thus covering the northwestern 

 end of the Carrizo Valley and the northern flank of the San 

 Jose Range, west to Creston and beyond. This district in- 

 cludes, therefore, the southern border of what is sometimes 

 known as the Estrella Valley, and is included within the drain- 

 age of the San Juan and the Estrella, a main tributary of the 

 Salinas River. The area lies intermediate between that of the 

 San Luis Folio mapped by H. W. Fairbanks, and that of the 

 McKittrick-Sunset District mapped by Arnold and Johnson. 

 The areal geology is therefore in a measure tied to each of 

 these areas. In a topographic and in a structural sense i't lies 

 between two of the more important ranges of this region, 

 namely the San Jose and the Temblor ranges. The former 

 borders it on the southwest as a massive abutment, and the 

 latter, as the southern unit of the Mount Diablo Range, sep- 

 arating it from the Great Valley. It may be considered as 

 belonging to the southeastern end of the Salinas branch of the 

 Temblor Basin, with which it was almost solely connected in 

 later Tertiary time. (See map, Plate 10). 



