DIKES OBSERVED ELSEWHERE BY AUTHOR 



253 



thin, ranging in thickness from mere films i to i ^inch thick, some of 

 them as much as 2 or 3 inches thick. The relations of two of these small 

 dikes are shown in £gsf 



mst 



figure 19. The dikes 

 shown in the figure 

 illustrate the occur- 

 rence in this district. 

 The sandstone of 

 the dikes is similar 

 to that of the over- 

 lying beds. The ori- 

 gin of these small 

 dikes is apparent. 

 Thejoint cracks were 



Figure 19, 



-Diagram of Sandstone Dikes near Stanford University, 

 California. 



formed in the basalt These dikes fill fissures along joint planes in basalt. The sand of the 

 ITT , i , dikes came in from above. 



probably as that ma- 

 terial cooled, and these cracks were afterward filled by sand from above, 

 probably with the aid of water, and at the same time that the overlying 

 sands were being deposited. 



NEAR MORRILTON, ARKANSAS 



A dike of fine grained sandstone intersects Carboniferous shales on 

 the east bank of the Arkansas river, just below the ferry, 2 miles south 

 of Morrilton, Arkansas. In 1891, when observed by the writer, this 

 dike was exposed for a horizontal distance of about 30 feet. It is but a 

 few inches thick, stands at a high angle, and its contacts with the shales 

 are smooth and sharply defined. 



The region is one of interbedded sandstones and shales, and there is 

 nothing to indicate whether the sand in the fissure came from above or 

 below. 



AT ZA VKERODA, SAXONY 



At the Oppel shaft of the Royal coal mines, at Zaukeroda, near Dresden, 

 Saxony, there are great numbers of dikes of shale and sandstone inter- 

 secting the coal beds, and in some cases the overlying shales, in all 

 directions. The dikes vary in thickness from a fraction of an inch to 

 60 or 75 feet (reported), and in some places they are so numerous as to 

 make up a considerable portion of the mass of the coal beds. The coal 

 bed, which has shale above and shale and sandstone below, is from 12 

 to 20 feet thick, and this entire thickness is often cut across by dikes 

 which branch, thin out, and intersect each other, and often form a per- 

 fect network, seriously interfering with the economical extraction of the 



