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



A careful measurement of the thickness of these beds is 

 difficult on account of numerous folds in the formation. The 

 minimum thickness is estimated at 1500 feet and in many- 

 places it cannot be less than 2500 feet. 



The correlation of these beds with the Santa Margarita 

 formation need not depend alone upon the invertebrate fossils 

 found in them. The character of the sediments and their 

 stratigraphic position is almost as convincing as their fauna. 

 A careful working out of the faunal zones in the Santa Mar- 

 garita formation would doubtless enable us to correlate them 

 with great accuracy as to detail. However, as this has not 

 been possible in the present work the writers are not able to 

 give an exact paleontological correlation of their various mem- 

 bers. There is some stratigraphic evidence, however, that is 

 worth consideration. The Santa Margarita in its type locality 

 lies unconformably upon the Monterey group. This uncon- 

 formity is marked by the presence of numerous angular frag- 

 ments of Monterey shale in the basal beds and also, in some 

 localities by a discordance in the attitude of the strata. In 

 some parts of the San Juan district the Santa Margarita is 

 apparently conformable upon the underlying Monterey, and 

 the separation between the two is made largely upon the litho- 

 logic characters. In the Mount Diablo region the relations of 

 the San Pablo formation to the underlying Contra Costa 

 County Miocene, the middle and upper parts of which are 

 probably the equivalent of the Monterey, while in some places 

 obscure, and in general apparently conformable, there are 

 nevertheless a few localities where there is evidence of uncon- 

 formity. Further south in the Mount Diablo Range there is 

 quite definite evidence in certain districts of unconformity be- 

 tween the Santa Margarita and Monterey Shale. On the east- 

 ward slopes of the Santa Lucia Range, west of the Salinas 

 Valley, the Santa Margarita is certainly unconformable upon 

 the Monterey in numerous localities. 



It thus appears that during the deposition of the Santa 

 Margarita vertical movements took place along the Santa Lucia 

 Range, and probably likewise in the Mount Diablo Range, 

 allowing a portion of the basin of deposition to be raised above 

 sea level while the area now occupied by the San Juan district 

 remained undisturbed and received continuous deposits. These 



