1:20 J. S. DILLEB I.NDSTONE DIKES. 



At 32 a L-fool dike cuts a bluff of conglomerate. Its strike is N. 38° E., 

 and on ascending it offsets to the northwest. At 33 are two dikes, one 14 

 inches and the other 6 inches through, while the dike at -'54 has a diameter 

 of 16 inches. At 35 a 12-inch dike appears and continues through three 

 exposures, the last one at 36. 



The linal dike of the series on Middle fork occurs at 37, just below the 

 cabin of J. ( '. Crow, two and one-quarter miles below the road crossing, and 

 is 1 foot in diameter. The search for <lik<- was continued over three miles 

 further up Middle fork, but none were found. 



Dikes on l)nj < 'reek. — On Dry creek more than twenty dikes are exposed — 

 a larger number than on any other stream, and they arc scattered over a 

 considerable distance. 



Just below the road crossing of I >ry creek, one and one-fourth miles above 

 the mouth of Salt creek, on the north hank of the stream, are four dikes 

 occurring at intervals for several hundred yards. The easternmost varies 

 in width from 14 inches below to only a few inches above. As it rises 

 through the shale bank twenty-five feet in height, it offsets several times to 

 the eastward. Near the base of the cliff there i< an offset of live feet, but 

 the two parts are partially connected. The shale- and sandstone beds at 

 this point strike N. 29 W. and dip 24° N. E. The rock of the dike is a 

 fine-grained sandstone, containing some mica and fragments of shale. 



The next vein, three hundred feet from the first, is about a foot in thick- 

 ness and strikes N. 13 E., with a slight dip to the N. \\\ It is wider below 

 than above, where it cuts a number of very distinct Bandstone Layers without 

 faulting. 



The third vein is onlv * inches through, and strikes N. 33° E., dips 85 

 N. W. It is very compact and onsets, as do its neighbors, to the Bouth- 

 i astward. 



The fourth dike varies greatly in width, from 14 inches below to •"> inches 

 in the middle, and then widens, with offsets, to 1 fool above. 



At 39, by the road in the stream bed, is a 20-inch dike exposed for over one 



hundred feet. It IS very regular, has laminated sides, and the middle por- 

 tion, as in nearly all the other dikes, is broken into approximately rectan- 

 gular blocks by the cross-fractures. 



A short distance above the road a prominent dike appears on the south 



I right) bank. It is only a foot thick but very Like a wall, as may hi seen 



in the accompanying illustration, plate 6, figure I, where a lateral view brings 

 out the cross-fractures very distinctly. It will be seen thai the transverse 

 joints are arranged in systems. All those of the same system are approxi- 

 mately parallel and cut across those of other systems in a manner quite un- 

 like the columnar jointing in dikes of igneous rocks. The greater number 

 of the cross-joints in this dike are horizontal, but a number are apparently 

 parallel to the beds of shale in the adjacent exposure. 



