SANDSTONE DIKES OF MISSISSIPPI. I 11 



the quartz, and shows slight traces of lateral crushing. In many cases the 

 folia are parted and the space between them occupied by cement, as in the 

 California dike rock. The alignment of the particles and their distortion is 

 not quite as conspicuous as in the dikes already described, but yet it is 

 sufficient to clearly indicate the character of the movement by means of 

 which the fissures were filled with sand. 



Of the country rock examined, none of the samples closely resemble the 

 sandstone of the dikes which, like that in California, is quite constant in its 

 character. One specimen of the four from the Eocene Buhrstone of the 

 same locality contains scales of muscovite, but the rock generally is of 

 finer texture than that in the dikes. 



Summary. • 



The sandstone dikes upon the forks of Cottonwood creek along the north- 

 western border of the Sacramento valley in California are over forty five in 

 number, and crop out at about 112 exposures throughout an area fifteen 

 miles in length from north to south and six miles in average width. 



They are all approximately parallel, with an average strike throughout 

 the whole area of N. 44° E. 



They are usually vertical, ranging from a mere film to 8 feet in thickness 

 and from 200 yards to 9i miles in length. 



They intersect the Cretaceous sandstones and shales along joints, without 

 distortion or displacement of the strata, and occasionally include numerous 

 fragments of the shale. 



They are sometimes banded vertically parallel to their sides, and the scales 

 of mica and other lamellar fragments usually stand on edge. in the same 

 plane. 



The dikes are traversed by joints in two principal directions, parallel and 

 transverse. Unlike the columnar jointing iu igneous dikes, the groups of 

 transverse joints in the sandstone dikes cross one another directly ; and the 

 principal group is usually parallel to the stratification of the adjoining 

 shales. 



The dike rock is an impure quartz sandstone containing considerable 

 biotite. The structure of the rock is unquestionably fragmental, and shows 

 no trace of crystallization in place of any material excepting the cement, 

 which is carbonate of lime. 



Much of the biotite is crushed in the direction of foliation, that is verti- 

 cally in the dike, since the scales stand on edge and the distortion of the 

 particles is such as to indicate that the sand moved upward in filling the 

 fissure. 



Filling fissures in the earth with sand from below is a common consequence 



