CONCENTRIC STRUCTURE OF THE SCHISTS. 209 



east and west direction. The change in the strike of the schist is not abrupt, 



as might be supposed from the above, hut in turning from one cardinal 

 direction to the next all intermediate positions of it are found. The schi.-ts 

 then form a broad concentric shell about the granite area. In going north 

 the schistose structure parallel to the granite becomes less and less prominent. 

 A lew miles away from it the rocks are found to have a structure parallel 

 to the granite and also one parallel to the slaty rocks to the north, the two 

 being nearly at right angles to each other. Going still farther away from 

 the granite, the slaty structure becomes more and more prominent, until 

 finally the schistose structure parallel to the granite has wholly disappeared. 

 In this pas-age the rocks have lithologicallv changed their character. Ad- 

 jacent to the granites they are completely crystalline. They become grad- 

 ually less and less crystalline as this rock becomes more remote, until they 

 merge into the unmistakable fragmental slates to the north, gaining the 

 north and south slaty cleavage in proportion as the schistose structure is lost.* 



The significance of the foregoing remarkable structural relations do not 

 seem to have struck either Newton or Crosby. It would seem that it is fatal 

 to the idea that the schistose structure represents bedding. It is, however, 

 at once explained by supposing the granite to be igneous. The parting and 

 crystalline character would then be regarded as due to contact action and 

 dynamic metamorphism. This suggested origin of the granite will be dis- 

 cussed later. 



We now have reached some conclusions as to the crystalline schists which 

 differ from Newton's. Instead of being in a definitely defined area in the 

 southwestern part of the pre-Cambrian core, they are in two areas, one about 

 the granites to the south, and the other about the eruptives to the north. 

 Nowhere was found a sharp boundary line between the schists and the slates. 

 The evidence which Newton gave for the existence of two series he states to 

 be mainly lithological ; also he says that " The line of separation between 

 them can be only imperfectly indicated. Its trend, so far as could be ascer- 

 tained, is a little west of north. "f The difficulty in the location of this line 

 is a direct sequence of the fact that the slates grade into the schists. Mr. 

 Caswell, who did the microscopic work for Newton, clearly appreciated that 

 in mineral composition these two classes are essentially alike. On this point 

 Newton says : J 



•• .Mr. Caswell's examinations show that the same minerals constitute the typical 

 rocks of both series, only in the schists they are more coarsely crystallized, so that 

 the lithological contrast seems to depend more on the degree or character of their 

 metamorphism than on any difference in chemical constitution/' 



* It would be of interest to ascertain the relations of the strikes and dips of the schists of the 

 extreme southern part of the pre-Cambrian area both to the smaller masses of granite and to the 

 Harney i>eak mass. 



(■Geology of the Black Hills of Dakota, p. 54. 



| [bid,, p. 62. 



