1888.] 491 [Crosby. 



been observed to cross that boundary in any case. I may add, as 

 the result of ray own observations, that the occurrence in the con- 

 glomerate of the eastern series of pebbles which have quite cer- 

 tainly been derived from the harder rocks of the western series 

 seems to leave little room for doubt as to the relative ages of the 

 two formations. 



Older or Western Series, 



Mica schist is the prevailing and most characteristic rock ot the 

 western or older series. Although subject to great variations in 

 character, it is in the main a gray and finely, but distinctly, crys- 

 talline rock. Garnet is the chief accessory mineral, and very fre- 

 quently, especially in the western part of the area, it is so highly 

 charged with garnets that it acquires a dark reddish color. The 

 garnets are well crystallized, but of small size, rarely exceeding a 

 quarter of an inch in diameter. The typical mica schist sometimes 

 changes to a distinct hydromica schist, which is also often highly 

 garnetiferous. It also passes imperceptibly into highly siliceous 

 or quartzose varieties containing but little mica, and from these 

 into a hard, tough quartzite, which is usually slightly micaceous, 

 varies from white to a light or dark gray color, and is bright and 

 glassy on the conchoidal fracture. The mica schists also pass 

 occasionally into chlorite schist and hornblende schist and less 

 rarely into gneiss. The gneiss, which is always highly micaceous, 

 is found chiefly in the granitic district about Harney Peak ; but 

 it is not a prominent rock even there. The mica schists are 

 traversed conformably with the stratification by many rather small 

 veins of quartz, which have usually a swelling or lenticular form. 



The granites of the Black Hills occur, as already mentioned, 

 only in the schists and gneiss of the western series. They are 

 found over a large area entirely in the southern portion of the 

 Archaean district. All of the granite masses which have been ex- 

 amined are parallel with the stratification of the schistose rocks ; 

 and when their structure is discernible they are seen to be, like 

 the quartz veins, lenticular in shape. 



Concerning the stratigraphy of the western series, much re- 

 mains to be determined. It is quite plain, however, that in a gen- 

 eral way at least the strike of the schists curves around the gran- 

 itic and gneissic area, and that the normal dip of the strata is away 

 from this nucleal mass at very high angles. 



