212 GEOLOGY OF THE BLACK HILLS. 



to the post-Cretaceous displacement. Here too the contrast of hard and 

 soft has determined the main features of the topography. The strata have 

 a nearly vertical position and trend obliquely across the area of their 

 exposure. Their hard members are quartzites; their soft, slates and schists. 

 Tlio former stand in ridges ; the latter underlie valleys. 



Within the newer series of rocks there are masses of trachyte, exotic 

 bosses, forming no part of the sedimentary series ; within the older series 

 are unstratified masses of granite, lenticular in form and exotic in origin. 

 The trachyte is harder than any of the sedimentary rocks ; the granite 

 rivals the quartzite in durability. Wherever these intrusive bodies have 

 been laid bare in the progress of the degradation they form eminences; 

 peaks in the case of the trachyte, ridges in tlie case of the granite. 



In this manner the main topographic features of the Hills have been 

 made to give ex])ression to the main facts of the geological structure. The 

 eminences point out the hard rocks; the valleys the soft. The arrangement 

 of hill and valley betrays to the geologist the character of the displace- 

 ments and the position of igneous masses. When, however, we leave the 

 consideration of the main features and descend to details, we find that the 

 arrangement of the lines of drainage has had much to do with the determi- 

 nation of the surface features. 



The drains as a rule cross the strike of outcropping strata at great 

 angles, often at right angles, and wherever a ridge dependent upon the rock 

 structure is intersected by a line of drainage it is cleft to its base. The 

 Carboniferous ridge which curves about the eastern side of the Hills is 

 interrupted in a distance of ninety miles by no less than fifteen creeks. 

 The Dakota ridge in making the circle of the Hills is crossed by twenty- 

 seven waterways. At every such intersection the absolute continuity of 

 the ridge is broken, but the interruption is so brief that the imagination fills 

 the hiatus without effort, and the structural continuity of the ridge is not 

 called in question. Indeed, if one who stands at the side of a ridge looks 

 obliquely down its course he finds it a matter of difficulty to discover the 

 points where it is divided. So, too, in the Archaean district ; the streams 

 intersect the quartzite ridges at high angles and divide them by narrow 

 canons, interrui)ting their continuity but not otherwise impairing their ridge 



