50 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Structure. — The structural features of the San Juan dis- 

 trict are best explained and understood with respect to the 

 topography, geology, and dynamic history of the surrounding 

 ranges and of the region. 



The district lying, as has been explained, between the San 

 Jose Range on the southwest and the Temblor Range on the 

 northeast, occupies a zone of low foot hills of younger strata 

 intervening and bordering upon both. The dynamic agency 

 that has effected the uplift of these ranges in earlier epochs has 

 been regional and compressive, acting from southwest to north- 

 east, in a direction at right angles to their trend. If at one 

 effort or epoch it has elevated the main ranges, or initiated the 

 main folds of the region, at later epochs it has developed the 

 minor folds in the younger and more yielding strata lying 

 between. 



The final result of the thrust movement as expressed in the 

 Tertiary strata along th*e San Juan River and throughout the 

 district is the development of a number of local and discon- 

 tinuous anticlines and synclines within the general area of the 

 trough, which therefore might be called a synclinorium. 



Along the southwest border of the sedimentary area the 

 strata lie upon the granitic basement dipping away at angles 

 between 10° and 30° toward the northeast. 



In the Santa Margarita formation which is the only one 

 exposed over a large part of the district, three or four sharp 

 anticlinal folds have been formed having northwest and south- 

 east axes, that continue longitudinally for limited distances 

 and then plunge or disappear into local synclines with which 

 they are in alignment. The dip on either side of the folds is 

 often so steep as to approach the vertical, and the transverse 

 shortening of the section from northeast to southwest must 

 have been considerable. This area of intense folding extends 

 for only a limited distance to the northwest toward the Salinas 

 Valley, while to the southeast it may be followed toward the 

 closely folded area of the Cuyama River and beyond. 



Of the anticlines developed in this area the most conspicuous 

 and persistent is that lying along the western border of the 

 Carrizo Plains, extending from the north side of T. 30 S., R. 

 18 E., toward the northwest for 15 or more miles, disappear- 

 ing in the northern part of T. 27 S.,'R. 16 E. This fold in- 



