l-*! v .1. S. Mt.l.Ki; INDSTOXE DIKES 



lit' the dikes represents the western limit of the newer formations of the Sac- 

 ramento valley, beneath which some of the dikes disappear. The largest 

 • like (in the North fork stands alone. The three large ones on Crow creek 



■ 



have been connected with their Bmaller representatives on the North fork a 

 mile above Gas Point. The Great dike extends from Roaring river to Dry 

 creek, a distance of 93 miles. Near the western border of the Greal dike 



on .Middle fork, at the dam, is another dike 5 feet in thickness, but it is com- 

 paratively short. Three dikes on Dry creek, Fight gulch, and Salt creek 

 have been connected as the exposures appear t<> warrant. These dikes are 

 thickeron Fight gulch than on Dry creek, indicating that they are thinning 

 out and probably do not extend very far beneath the newer formations. 



The general parallelism of the dikes is well shown, although there is a 

 divergence of 51 degrees iu their strike, ranging from N. 20° to 71° E. The 

 average strike on the Ninth fork is N. 47° !•'..: on Crow creek, N. 54° E.; 

 on Squaw creek, N. 53 E. ; on Roaring river, N. 54° E.; in Poverty gulch 

 N. 43 E. : in Aiken gulch, N. 40° E. : on Middle fork, N. 4"J E. . on Dry 

 creek, N. 10 E.; in Fight gulch, N. I" E.; on Salt .reek. N. 34° E.; on 

 the Stage road, N. 38 E. The average strike of all north of Aiken gulch 

 is N. 4i>° E., and south of it N. 39° E. The more easterly trend of the 

 northern dikes may he -ecu in the accompanying map. The same bending 

 to the eastward may be observed in the Great dike. On Roaring river its 

 average strike is N. 57 E., the lowest being N. 48° E. On Middle fork 

 its average is N. 11 E., and the highest N. 4-">° E. . 



If we regard these dikes as earthquake phenomena their gentle curvature 

 may indicate their relation to the center of disturbance far to the southeast- 

 ward in the Sacramento valley. The assures do not appear to belong to the 

 Sonora or Owen's valley type, in which case the lissures follow the base of a 

 mountain range and are associated with faulting. In this case the fissures 

 are some distance from the base of the Coasl Range, and no faulting has 

 been observed. 



Oro8by'8 Theory of the Origin of parallel Joints. — The theory proposed by 

 M r. W. < >. ( irosby to explain the origin of parallel joints is of interest in 

 this connection. He regards them as fractures produced by earthquakes, 

 and the theory has much in it> favor. It is strongly supported by the 

 phenomena here described. The joints in the gh ales are generally most 

 noticeable in the neighborhood of the dikes, ami the dikes themselves occupy 

 joint fissures which must have been formed at about the same time and by 

 the Bame general movement a- the dikes. Wide fissures, it' hit empty in boA 

 strata under pressure, would not remain "pen ; their Bides would gradually 

 come together. 



Pi ledlngs Boston Sex Sal History, vol. XXII, p 72 



