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



covered by well stratified beds of the following group in which 

 freshwater shells have been found plentifully in certain locali- 

 ties. 



Paso Roblcs Formation. — The Santa Margarita is overlaid 

 unconformably by a series of gravelly sands and sandy clays 

 that are, in part, at least a portion of the formation which is 

 prominently developed in the Salinas Valley, and which has 

 there been called by the name, Paso Robles formation, and has 

 been supposed to be of freshwater origin. 



The limits and distribution of this formation along the San 

 Juan River are rather obscure, and it is difficult to separate it 

 from the stream gravels and alluvium which is believed to be 

 largely of Quaternary age. In the canyons east of the San 

 Juan River, and dipping at a considerable angle westerly are 

 sandstones and gravels with interstratified beds of clay which 

 rest upon strata of possible Etchegoin age. These overlying 

 strata contain numerous shells of freshwater mollusks. Among 

 the species collected from these beds are 



Lymncea ciihensis Lea, 



Lymncua, of. obrussa Say, 



Physa heterostropha Hald?, and 



Planorhis, sp. 



Along the northern and eastern parts of the district the 

 Miocene is extensively covered by the Paso Robles formation 

 which appears to have once extended entirely across the range 

 in the vicinity of Polonia Pass, and to have been connected with 

 similar beds in the Great Valley. 



In former papers these Paso Robles beds have been corre- 

 lated^ with the Tulare group which is known to extend into 

 the Antelope Valley on the west side of the Temblor Range. 



Stream Gravels. — Overlying all of the older formations and 

 groups from the basement rocks to the Paso Robles there are 

 beds of stream gravels and alluvium widely spread over the 

 entire district. These gravels mantle large areas of the Mio- 

 cene in the central and eastern half of the San Juan district. 

 They consist of pebbles of quartzite, sandstone, limestone, 

 granite and basalt. They cover the Miocene rocks in all the 

 higher portions of the eastern part of the district to a depth 

 of from one foot to 200 or 300 feet. In general, the Miocene 



^ Proc. Calif. Acad. Sci., Vol. Ill, page 32. 



