130 .1. S. MI.I.Kl; ANDSTONE DIKJ - 



Description <>{ Specinn m. 



No. 1. Sandstone dike on Sail creek, \ mile above McNett's. 

 *_'. '■ 1 i miles below Ono bridge, on North 



fork of ( lottonwood. 

 •">. '• | mile below John Allen's, on Dry 



creek. 



i n .. .. .. .. it 



al John Allen'.-, on Dry creek. 

 6. bed at dam on Middle fork, 1 mile above 



Miller's. 

 i. top of cascade, 3 mile up Byron creek 



from ( >no. 

 • s . " 21 mile> above Johb Allen'.-, on Dry 



creek. " 44. 



Of the foregoing analyses numbers 1. 2, 6, and 7 were kindly made for me 

 by Mr. Thomas M. Cbatard, and numbers ■">. I, 5, and * by Mr. J. Edward 

 Whitfield, in the chemical laboratory of the I.'. 8. Geological Survey. 



The range of silica in the dike rocks is from 18.10 to 61.60, while in the bed 

 rock it is from 55.85 to 67.62, with a considerably higher average amount 

 than in the dike rock-. The same is true to some extent of the oxides of 

 iron. These are fully counterbalanced by the lime and carbon dioxide, 

 which shows that the lime carbonate is more abundant in the dikes than in 

 the beds— a fact which is apparent also under the microscope. The carbon- 

 ate of lime i- the ,•<• nt ami. being a .-< idary deposit, should not he con- 

 sidered a constituent of the original sand. It bas already been shown that 

 in mineralogical composition the dikes ami certain bed rocks are practically 

 identical, and the chemical analyses illustrate the same fact 



Geologic Relations \m> Origis of the Sandstone Dikee 



Position "a 'I Age. -From the geologic map, figure 2, in which the distribu- 

 tion of the .like- is shown, il will he -ecu thai they are confined to the 

 Cretaceous Horsetown and Chico beds. Figure 8 is a cro — ectionofthe 

 same region, ami shows in a general way the relations of the rock- from 



Bully Choop in the Coast Rang i the northwest, to the Sacramento valley 



on the southeast. Thej arc naturally separated into four groups of for- 

 mations: i 1 fhc Metamorphic rocks of the Coast Range; (2) the Cretaceous 

 formations of the Bald hills, which arc marked by an old base level of erosion 

 and composed of conglomerates, sandstones, and -hale-; of tic Horsetown 

 , and Chico beds ; (3) the sandstone dike- which penetrate these beds; ami 

 I i the tuff, gravels, sands, and clays of the newer formations which lie in 

 the Sacramento vallev. 



