1916 J 



Dickerson : Tejon Eocene of California 



383 



a thick basal conglomerate resting unconformably upon Chico-Mar- 

 tinez rocks, white to dull red sandstone with subordinate shale beds, 

 and three coal seams interbedded with soft shales and white sand- 

 stones. Its upper limits have not been certainly discovered, but 

 there is a sudden change in lithology north of the village of Somers- 

 ville which may mark a division line between it and the overlying 

 brown carbonaceous sandstones of possible Oligocene age. 



The Martinez group in this vicinity is represented surficially by 

 a strip averaging one-quarter of a mile wide which extends from 

 lower Oil Creek westward for four miles. Its west end is terminated 

 by a cross-fault, while its eastern end is cut off by the Tejon con- 

 glomerate. 



Throughout the area studied there is a constant difference in 

 strike between the Martinez and the Tejon. This is generally ten 

 degrees, and in lower Oil Canon it is much greater. This difference 

 in strike causes the Tejon conglomerate to rest upon a stratum of 

 hard Martinez sandstone at one locality and upon soft Martinez 

 shales at another. This accounts for a very irregular Martinez- 

 Tejon contact. 



The dip of the Martinez throughout the field is greater than 

 that of the Tejon. The basal Tejon conglomerate, which is from 

 ten to twenty feet thick, rests upon the Martinez sandstones and 

 shales and forms a well-defined bed for over four miles in length. It 

 consists of very coarse pebbles and boulders, which make it easily 

 separable from the sandstones of the Martinez. The pebbles and 

 boulders are in most places quartzose, but fragments of fossiliferous 

 limestone, and sandstone and igneous rocks also occur. Nucula 

 truncata, Cylichna eostata. Modiolus cylindrieus( ? ) , Dentalium 

 cooperii, and mollusc-bored boulders which resemble very closely the 

 pholad borings on the Martinez- Chico contact described below, have 

 been obtained from limestone and sandstone boulders imbedded in 

 the conglomerate. Either the Chico or Martinez groups supplied 

 this material, or possibly both may have contributed to this basal 

 Tejon, as these species are limited to the upper Cretaceous and the 

 Eocene on this coast. 



The evidence of unconformity between the Martinez and the 

 Tejon groups in the region north of Mount Diablo is based (1) upon 

 area! mapping of the beds containing characteristic faunas of these 

 groups, (2) upon variation of strike at the contact, (3) upon varia- 

 tion in dip throughout the area studied; (4) upon the presence of 



