THE GREAT DIKE. 417 



other dikes. Although the fractures are nearly all transverse, cutting the 

 dike into irregular blocks or columns, there are a few fractures near the 

 edge of the dike parallel to its sides. 



Fifty yards west of the large dike here exposed are the two small ones 

 shown in plate 6, figure 1. The larger one on the left is a foot in diameter, 

 and has well-developed parallel jointing. An important relation of the 

 principal set of transverse joints to the bedding in the shale is well illustrated 

 in these dikes, where it is seen that the stratification and the most con- 

 spicuous cross-jointing are parallel. Specimens 1971 and 2384 from the 

 middle portion of the Great dike are apparently coarser grained ; 1970, 

 2385, and 2386 are from the more compact and apparently finer-grained 

 border. Specimen 2387 is from a little dike close by the great one, and 

 2388, 2389, and 2390 are from the two dikes 50 feet away. 



The Great dike continues southwest across a bend of the stream, and is 

 well exposed at 10, where plate 8 represents its appearauce. It is here 5 

 feet in greatest width, and divides downwards into a number of smaller dikes. 

 The finer-grained and somewhat harder edges of the mass and its cross- 

 fractures are here well exposed. Within the shadow in the central portion 

 of the dike there is an inclusion of shale. This included shale is soft and 

 spheroidally weathered, exactly like that upon the sides of the sandstone 

 dike. Scarcely a trace of jointing can be detected in the adjacent shales at 

 this point, but at a few other localities it has been observed in connection 

 Avith the dikes. The direction of the bluff here is such that the shales appear 

 to be horizontal, but in reality they are slightly inclined. Specimen 2391 

 is from the lower portion of this dike, and 2392 from the included shale. 

 Near this exposure the shales strike N. 10° E. and dip 17° to the eastward. 



Continuing southwestward, the Great dike crosses another elbow of the 

 stream and is again exposed at 11, in an abandoned placer mine, where it 

 is 3 feet thick. Its dip is 82° S. E. and its strike N. 48° E., which carries 

 it across the divide to Poverty gulch near Mr. Glass's, 2 miles away, where 

 it again appears. Associated with it at 10, on Roaring river, are several 

 smaller dikes. One is 6 inches thick ; strike N. 47° E. Another is 1 foot 

 through, and dips 77° N. W. A third is only 2 inches in thickness. Dis- 

 tinct traces of joints are developed here, and their strikes and dips are the 

 same as those of the dikes ; furthermore, they appear to occur in the neigh- 

 borhood of the dikes only. In fact, some of the joint-cracks which escape 

 sight at a first hasty glance, when examined more carefully are found to be 

 filled with sand in all respects like that of the larger dikes with which they 

 are associated. Chips may with difficulty be obtained showing one of these 

 miniature dikes, but generally the intruded sand of the dikes separates very 

 easily from the adjacent shales, and thin sections of the contact cannot be 

 obtained. 



