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U)iivc7-sify of California Publications in Geology [Vol. 12 



CEETACEOUS SYSTEM 



Shasta and Criico Rocks 



The Cretaceous strata resting uneonformably upon the Franciscan 

 series can be separated into two divisions : first, a lower series consist- 

 ing mainly of shale interbedded with thin sandstones and a small 

 amount of conglomerate; and secondly, an upper series of strata 

 made up. for tlie most part, of sandstone with minor intercalations 

 of shale and conglomerate. An unconformity between these two 

 series is suggested by the presence of a heavy conglomerate at the 

 base of the \ipper member, together with the fact that there is a sharp 

 change in lithology from beds below the conglomerate to those lying 

 above, and a slightly irregular contact between them. The lower 

 beds are definitely known to be of Knoxville (Lower Cretaceous) age 

 since Aucella pioclti (Gabb) occurs abundantly in them. The upper 

 beds may represent the Chi(>o (Upper Cretaceous), though no fossils 

 have been found to prove tliis statement. 



Tn detail, the lower or shale strata consist of greenish black shale 

 with minor amounts of interbedded sandstone and limestone. The 

 sandstone is of a lighter color, fine to moderately coarse grained. 

 The beds of limestone are quite impure, black to dark in color, 

 and six to nine inches thick. About 300 feet stratigraphically above 

 the base is a fifteen-foot bed of fine conglomerate, the pebbles of 

 which average about one-half inch in diameter, though some are as 

 large as a hen's egg. They are derived mainly from quartzite, black 

 chert, and igneous rock. The upper fifty feet of the shale member 

 consists of coarse sandstone formed of subangular grains of quartz, 

 biotite, and serpentine. Marine fossils are present though indeter- 

 minable. All the bedded shaley strata are greatly folded and faulted, 

 so that in many places the deformation resembles that occurring in the 

 sedimentary beds of the Franciscan series. 



The heavy conglomerate at the base of the upper series is somewhat 

 lenticular in character, though it can be traced from the mouth of 

 Blue Caiion along the ridge south of Gamusa Cafion ; it again appears 

 on the south slope of Little Pine Mountain immediately west of the 

 Oso Canon trail. A noticeable feature of this is that it occupies 

 the same horizon as the Oakland conglomerate on the east side of 

 San Francisco Bay. This conglomerate is made up of well rounded 



