62 GEOLOGY OF THE BLACK HILLS. 



homogeneous soft beds; and the presumption is that no such folding- took 

 place within the area exposed in the Hills. The whole system of vertical 

 beds, with a width of about twenty-five miles, is believed to retain its origi- 

 nal relation of parts. It has not, of course, its original position, for the 

 same great process of change which has produced its metamorphic structure 

 has turned it bodily on edge and either broken away or eroded away its 

 upward continuation; but it is probable that the system presents the clays 

 and shales and sandstones from which it was produced by metamorphism 

 in the same order in which they were originally deposited. 



The division of the system into two series, a series of schists and a 

 series of slates, is based on lithological differences purely, and is fully war- 

 ranted, whatever may be the structural and historical relation of the two. 

 The fact that Mr. Jenney discovered an apparent unconformity between 

 them at one point, although it stands alone, and although the two series 

 generally conform in strike, is not to be ignored ; and coupled with the 

 great lithological difference gives strong support to the view that the slate 

 period and the schist period were separated by an interval of time and not 

 merely distinguished by a change of sediments. Indeed it is questionable 

 if the original sediments were materially different. Mr. Caswell's exami- 

 nations 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. This 

 fact gives additional support to the view that there was an intervening 

 lapse of time; for if we assume that the schists are older than the slates, it 

 is but natural that they should have been subjected to an antecedent and 

 more powerful metamorphism from which the slates were exempt. 



My idea of the history, not by any means as proved, but as most 

 probable in the imperfect light of the facts now known, is as follows : The 

 sediments of the western series were first deposited and were altered to the 

 condition of schists. They were then raised above the ocean, were somewhat 

 eroded, and sank again. The eastern series were deposited on them with 

 slight unconformity, and the whole were again subjected to metamor- 

 phic action, which ceased when the eastern rocks had reached their present 



