STKATIGFvAPHY OF THK DIKE REGION. 



431 



The Cretaceous group of strata appears to be a continuous, conformable 

 series, thousands of feet in thickness. The basal bed, well exposed on Eagle, 

 Byron, and Jerusalem creeks, is a heavy conglomerate of coarse, round and 

 sub-angular fragments derived directly from the older metamorphic rocks, 

 upon which it rests unconformably, and marks approximately an ancient 

 shore line of Cretaceous time. 



The strata of the lower portion of the group lyiug on the North fork of 

 Cottonwood creek above the mouth of Hulen creek contains an abundance 

 and great variety of fossils, regarded by the California Geological Survey 

 and Dr. C. A. White as belonging either wholly or in large part to the 

 Horsetown beds. At the mouth of Hulen creek the Chico beds, characterized 

 by many fossils, occur and extend eastward, passing beneath the later for- 



Consl K.mgc 



r&menlo Valley 



Figure 8.— Section across the Dike Region along the North Fork of Cottonwood Creek, in Shasta 



County, California. 



l=Metamorphic and dioritic rocks of the Coast Range ; 2=Cretaceous conglomerates, sandstones, 

 and shales; 3=Sandstone dikes; 4=Newer formations of the Sacramento valley. There may be 

 an unconformity near the middle of the section. 



mations of the Sacramento valley. Near the western limit of the newer 

 formations, the Chico beds are penetrated by the sandstone dikes already 

 described. 



Their vertical position indicates that they were formed after the tilting of 

 the Chico strata, and the fact that they are overlapped by the Salt creek 

 and Tuscan formations demonstrates their existence before these formations 

 were developed. 



After the tilting of the Chico beds, their upturned edges were worn off to 

 a general level, a base level of erosion ; and in this process the tops of the 

 dikes were removed also, showing that they were formed between the times 

 of the tilting of the Chico group and the development of the base level of 

 erosion across the Cretaceous belt. The Chico beds are the top of the Creta- 

 ceous, and the dikes which penetrate them could not have been formed be- 

 fore the close of the Cretaceous. The formations of the Sacramento valley 

 which are younger than the dikes are Pleistocene, and in part probably 

 Neocene, rendering it altogether probable that the dikes were formed some- 

 time during the Eocene or Neocene. 



The Dikes occupy Joint Fissures. — Joints are uncommon and poorly de- 

 veloped in the Cretaceous shales. They were seen chiefly in the vicinity of 

 sandstone dikes. The latter occupy fissures between joint planes, and it is 

 evident that the position of the dikes was determined by the joints. It may 



