5i- 



A. G. LEONARD 



abundant at certain points. Some portions of the rock are so 

 completely rilled with these brown concretions that they constitute 

 the main bulk of the formation, and the gray, loosely cemented 

 sandstone forms a kind of matrix in which the hard nodules are 

 imbedded. In the process of weathering these more resistant 

 nodules project far beyond the softer rock, and at the base of slopes 

 and scattered over the surface they are exceedingly abundant. 



K~ 



Fig. i. — The Fox Hills sandstone on Cannon Ball River, North Dakota, showing 

 hard ledges and concretions on a weathered surface. 



Where the rock has only a few concretions, and therefore, where 

 the iron has not been segregated to as large an extent at certain 

 points, the sandstone is of a yellow color, due to the disseminated 

 iron oxide. On the other hand, where the brown ferruginous 

 nodules are thickly scattered through the beds, the rest of the rock 

 is gray, the iron having been largely leached from it and concen- 

 trated in the nodules. Many of the latter are of good size and 

 spherical in shape, and it is these which have given its name to the 



