DIKES FOUR AND FIVE FEET IN THICKNESS. 



415 



illustrated in figure 3. Its strike is N. 55° E., and its dip 82° N. W., pene- 

 trating the Cretaceous shales without faulting or indurating them in the 

 least. This exposure is of special importance in showing that the dike does 

 not penetrate the tuff and beds which lie beneath it upon the upturned shales- 



Dikes on Crow Creek. — Half a mille above the mouth of Squaw creek, at 

 4 on the map, is a 4-inch dike exhibiting good joints. Its strike is N. 71° 

 E. At 5, half a mile further up the stream, there is a well defined vertical 

 dike 1 foot in width ; strike N. 63° E. Near by is one 7 inches thick. Its 

 strike is N. 56° E., and with increased width (1 foot) it continues up stream 

 for several hundred yards. 



About 1 1 miles above the mouth of Squaw creek, at 6, is a group of prom- 

 inent dikes approaching the valley from the northeast. The first is about 

 2 feet in diameter, and the other three are about half as large. One of these 

 crossing the little valley enlarges and becomes 4 feet thick, and forms a 

 prominent, wall-like bluff twenty feet high, shown in plate 7, figure 1. 



Fkjure 3. — Section exposi d on the North Fork of Cottonwood Creek at Gas Point. 



A 14-inch sandstone dike (9) penetrates the Cretaceous shales (8), which are overlain unconform- 

 ably by the late formations(l-7) of the Sacramento valley. a=Sluice-box; l = Auriferous gravels of 

 Red Bluff formation ; 2=Tusean tuff, 3 feet; 3=Clay, 4 feet; 4= Irregular, fine yellowish gravel, 8 

 feet; 5=Tuff (?) ; 6= Irregular, reddish clay and sand, 12 feet; 7= Ferruginous gravel, sometimes 

 cemented, 12 feet; 8=Cretaeeous Shales (Chico); 9=Sandstone dike. 



The transverse cracks in this dike are parallel to the stratification in the 

 shale at the right. They so divide the dike into blocks that it resembles 

 courses of masonry. This resemblance has led many people of the district 

 to regard the dikes as ancient walls, perhaps of some prehistoric people. 

 This is the largest exposure of the kind seen in the country, and is well 

 known for the excellent shade it affords from the hot afternoon sun. 



Near by is another dike, 5 feet in thickness. Its strike is N. 40° E., and 

 it can be traced in that direction across the little vale to the hill a quarter 

 of a mile away. A short distance northwest of these dikes the valley of 



