GENERAL CHARACTERS OP SANDSTONE DIKES. 425 



The dike rocks frequently contain fragments of shale. They are generally 

 small, but occasionally as large as a* hand and rarely larger. The shale 

 fragments are usually flat and arranged with the scales of mica parallel to 

 the sides of the dike, but this is not always so, for they may be thick, angular, 

 and without orderly arraugement. 



A common phenomenon which, however, is not universal is that the sides 

 of the dike are more solid and apparently also of finer sand than the middle 

 portion. Occasionally, too, the dikes are distinctly banded near the edges, 

 and this banding is found to be due to streaks of finer and coarser sand ; but 

 it is not a conspicuous phenomenon. It may, however, be distinctly seen in 

 a hand specimen of the rock at a distance of twenty feet. 



A more important feature, and one which will be noticed more fully at 

 another place, is the arrangement of the scales of mica in the dike parallel 

 to its sides. In a few cases the mica scales had no definite position, but 

 generally they are arranged as indicated and give to the rock a direction of 

 easiest cleavage. 



The dikes have two sets of fractures, one transverse and the other parallel. 

 The transverse fractures divide the mass into more or less regular six-sided 

 blocks, giving the dike a rudely columnar appearance. It is generally true, 

 also, that the most abundant set of cross-joints is parallel to the stratifica- 

 tion of the adjoining shales. The other joints, which are parallel to the sides 

 of the dike, may be absent, and when present are usually most abundant 

 close to the border of the dike, imparting to it a lamellar structure. 



MlNERALOGICAL COMPOSITION AND MlNUTE STRUCTURE. 



These dike rocks are wonderfully uniform in physical properties and 

 composition throughout their whole extent. Upon a fresh fracture the color- 

 is gray, varying slightly in shade, but when weathered it is yellowish, owing 

 to the presence of iron oxide. 



Biotite appears to be always present, and generally in considerable quan- 

 tities, so that it is one of the first minerals recognized when examining the 

 hand specimen, but is not so abundant as to make the rock conspicuously 

 micaceous. The rock is too fine grained to allow a further determination of 

 the constituent minerals without the aid of a microscope. 



In the thin section the rock is seen to be composed largely of quartz, feld- 

 spar, and biotite, with considerable calcite cement. Serpentine, titanite, 

 magnetite, and zircon are less common, and other minerals are rare. Besides 

 the fragments of simple minerals, there are numerous composite grains 

 derived from metamorphic rocks. 



The grains of quartz are usually far more a'bundant than any other kind, 

 and constitute on an average (roughly estimated) about 40 per cent, of the 

 whole rock. They are commonly angular, and rarely well rounded. In 



