Vol. IV] ANDERSON AND MARTIN— NEOCENE RECORD 45 



The Monterey group, as in all other localities where it 

 occurs, is conspicuously unfossiliferous, except for the micro- 

 scopic organisms which are of little value for correlation. The 

 average thickness of the beds is between 600 and 1000 feet. 



The Santa Margarita Group. — Overlying the Monterey 

 group is a thick aggregate of strata consisting of gravelly 

 sands, conglomerates, granitic sandstones and sandy clays that 

 form a distinct group, lithologically. It is here classed as the 

 Santa Margarita group, though perhaps the equivalent of the 

 San Pablo of the Mount Diablo region. Although this group 

 of strata appears to have a much greater thickness and to con- 

 tain older beds than the Santa Margarita formation does in its 

 type locality, for reasons which will be given later, it has been 

 included entirely under this name. The Santa Margarita 

 group, as well shown toward the southern part of the district, 

 is unconformable upon the older groups, and although in some 

 localities its relations are obscure, the evidence of uncon- 

 formity is satisfactory in others. 



The Santa Margarita group is the most prominent in the 

 San Juan district on account of its thickness and great areal 

 distribution. In the western part of the district it occurs as 

 a long narrow zone parallel and adjacent to that of the 

 Monterey shale. In the eastern part of the area the Santa 

 Margarita outcrops in a rectangular zone which parallels and 

 flanks San Juan River. If the covering of stream gravels and 

 alluvium were removed from the surface, the Santa Margarita 

 would be found to occupy nearly half of the San Juan district 

 on the north and east. 



At the base of the series there is a thin zone of soft, sandy 

 clay shales which grade downward into the Monterey and up- 

 ward into medium grained sandstone which is followed above 

 by coarse arkosic sandstone and gravelly sands. The latter pre- 

 dominate throughout the formation, although there are numer- 

 ous layers of soft sandy clays interstratified with the coarser 

 sediments. East of the San Juan River, in the central part of 

 T. 29 S., R. 17 E., the formation is composed almost entirely 

 of coarse granitic material which is often so well cemented 

 by the lime from fossil invertebrates that it stands out prom- 

 inently on the hillsides in rugged outcrops. 



