Crosby.] 496 [March 7, 



each other, to the enormous pressure which wrought such a radi- 

 cal change in the structure of the associated conglomerate. The 

 quartzite has, like the conglomerate, been reduced horizontally and 

 squeezed out vertically. In the absence, however, of compressi- 

 ble pebbles or grains and of material from which hydromica might 

 be abundantly formed, it has failed to develop the coarsely schis- 

 tose structure of the conglomerate or the finely fissile structure or 

 slaty cleavage of the argillites and phyllites ; but the extent and 

 direction of the differential movements of its grains are simply in- 

 dicated by this rectilinear streaking. It is interesting to observe 

 also that a similar longitudinal gliding is often indicated in the ar- 

 gillites by parallel vertical striae or corrugations on the cleavage 

 surfaces. 



Correlation of the Archaean Formations. 



Mr. Newton has discussed at some length the question of the 

 geological age of the western and eastern series. He carefully 

 compared them with the Laurentian and Huronian systems re- 

 spectively, only to find that, while the eastern series bears some 

 resemblance to the Huronian, the western or older series is wholly 

 unlike the Laurentian, and that in neither case do the facts justify 

 even a provisional correlation. That the metamorphic rocks of 

 the Black Hills are truly Archaean, and were fully metamorphosed 

 before the dawn of Potsdam time, is proved by the Potsdam rest- 

 ing upon their upturned edges unconformably, and carrying in the 

 conglomerate at its base fragments of slates, schists, quartzites, 

 etc., of a character precisely similar to that of the underlying 

 rocks. 



The writer's observations very soon convinced him, however, 

 that Mr. Newton ought to have looked to New England, rather 

 than to Canada, for the equivalents of the Archaean strata of the 

 Black Hills. Although we may not, before the older rocks of the 

 west have been more thoroughly studied and described, insist upon 

 their exact chronologic correspondence with any formations in the 

 east, there can be no question that both the older and newer series 

 recognized below the Potsdam in the Black Hills are, lithologi- 

 cally, almost identical with two of the prevailing systems of met- 

 amorphic rocks in the eastern United States and especially in 

 New England. 



The newer series of slates, phyllites, hydromica schists and 



