64 GEOLOGY OF THE DLACK HILLS. 



thoiiy:li of considerable ina<rnitiide, are not frequently recurring. The col- 

 lection of the t}pical rocks of the slate series brought in from the Black 

 Hills have a j)eculiar resemblance in their general facies to the typical 

 Iluronian rocks, and this fact was inunediately noticed by several geolo- 

 gists to whom the Iluronian types are familiar. 



In the western series of schists we cannot trace so close a resemblance 

 to either member of the Archa'an of the East. The characters which dis- 

 tinguish them from the adjacent slate series serve equally to distinguish 

 them from the ty[)ical Iluronian. Their rocks are all found in some abun- 

 dance in the Laurentian, but they are not the characteristic rocks of the 

 Laurentian. If there is one Laurentian rock more prevalent than another 

 it is gneiss, but gneiss is almost unknown in the Black Hills ; while calca- 

 reous rocks and magnetic iron ores, the constant associates of the Lauren- 

 tian schists, do not occur in the Archaean of the Hills. 



The lithological evidence fails, therefore, to give even its feeble sup- 

 port to the theory that the two Archaean groups of the Black Hills are the 

 representatives of the two Archaean groups of Canada. The fact that there 

 are two great groups in each case is a resemblance. A second resemblance 

 is found when the younger group of the two pairs are compared with each 

 other. But when the older groups are com2)ared a decided disparity is 

 brouo^ht to lio-ht. With evidence of this character nothino- short of a full 

 coincidence could serve to establish even a presumptive correlation, and the 

 question can only await further light. I have, therefore, abstained from 

 any provisional assignment of names to the slates and the schists further 

 than to call the former Newer Archaan and the latter Older Archaan. 



'i'he Archaean rocks of the Black Hills, like Archaean rocks elsewhere, 

 were fully metamorphosed before the dawn of Potsdam time. This is 

 proved by the Potsdam resting upon their upturned edges unconformably, 

 and carrying in the conglomerate at its base fragments of slates, schists, 

 quartzites, etc., of a character ])recisely similar to that of the underlying 

 rocks. 



A portion at least of the Archaean area was above the sea in early 

 Potsdam time, for the Potsdam conglomerate could only have been formed 

 by the action of waves on a shore line. The erosion of the Ai'chaean, which 



