THE GEAXITES. 65 



ceased with the deposition of the Potsdam, was not renewed until after the 

 Tertiary disturbances which produced the Black Hills uplift. 



SECTION III. 



THE GEAXITE S. 



The granites occur only within the area of the Archaean ; but the dis- 

 cussion of their character and origin and the question of their age present 

 so many points of interest that it has been thought better to consider them 

 separately instead of treating them in connection with the slates and schists. 



As has been mentioned already, they are wholly confined to the south- 

 ern end of the Hills; none whatever are found north of Spring Creek. Their 

 greatest development appears in the accumulation of ridges and summits 

 about Harney Peak. North of Harney little granite is found and that 

 merely the ending off of the main range. West, south, and east it becomes 

 gradually less and less predominant and the schists are more and more 

 exposed, until at the limit of the Archaean exposure it is rarely met with. 

 West of Harney there are many masses of no little magnitude but they 

 are more abundant toward the south and southeast. In several of the 

 canons of the creeks draining the southeastern corner of the Hills granites 

 appear among the quartzites and schists, while the tops of the canon walls 

 are capped by the fossiliferous rocks. Harney thus seems to be an eccen- 

 tric point in the development, the northern focus of an ellipse whose longer 

 axis is directed to the southeast and whose area incloses all the outcrops.* 



The maximum expansion of the granites around Harney seems to be at 

 or near the separating line between the two series of metamorphic rocks, 

 the schists of the western and the slates of the eastern series, and our care- 

 ful examination found them to be confined entirely to the area of the schists, 

 none whatever being found in the slates. 



It would be a task of no little magnitude to give an adequate idea of the 



*Note BY THE EDITOR. — The reader who compares the distribution of the granite, as described 

 in the text, with its distribution as given by the geological map, 'will find discrepancies. The author 

 drew a few granite areas on a small preliminary map, but attempted no thorough representation, because 

 the topographical material was not then fully compiled. The editor has transferred these areas to the 

 final map, but has not attempted to supply the outcrops described in general terms in the text, because 

 he could not do so without inventing details. 



5 B H 



