'-Ml U. i;. VAN H.1SE — I'KI.-i A.MBRIAS <»l THE BLACK HILLS. 



ut' ih«- same character is at the base of the Wasatch series. This is a scarcely 

 less notable example, its lower parts resting under 30,000 feel of conformable 

 sediments.* Peldspathic detritus, while also exhibiting greal permanence 

 when not subject to powerful dynamic action, is not bo refractory as quartz, 

 [n the "slate conglomerates " of Logan and Murray on the north shore of 

 Lake Huron -a part of the "original Huronian " — the abundant feldspathic 

 debris ordinarily shows its orignal well-rounded forms. In the Penokee 

 series, just referred to, is a belt of mica-slates and mica-schists. These vary into 

 quartzose phases at various points, which show that they, like the quartzites, 

 are unmistakably offragmental origin. The feldspar has, however, locally 

 in large measure decomposed into quartz and mica, and in the few places 

 where it has been the predominant or sole mineral the decomposing processes 

 have bet n sufficient to obliterate the evidence of the original clastic char- 

 acter of the rock.i But, upon the whole, the permanency of fragmental 

 characters in rocks when simply upturned, nol folded, however old they may 

 be or however deep they may be buried, is astonishingly great. 



But the moment actual movement begins within a rock, evidence of frag- 

 mental origin is rapidly destroyed. For instance, the great mass of the 

 Devil's lake quartzites of central Wisconsin exhibit- perfectly, under the 

 microscope, its fragmental character, but alone- certain narrow zones slipping 

 action has taken place; the grains have here Keen elongated in a common 

 direction, and it is hard to find the original clastic <-<>vc< if they vet exist. 

 Movement within the mass of the rock has obliterated the evidence of its 

 fragmental origin. Of course this idea of obliteration of clastic character- 

 istics by rock movement is as old a- Dana's theory of metamorphism. I 

 wi8h, however, to emphasize their permanency when movement has not oc- 

 curred, although the rock may now be completely vitreous, crystalline, of 

 great age, and may have been subjected to enormous pressure. 



In the Black Hills dynamic action has extensively occurred. Crystalline 

 schists have been formed from unmistakable fragmental rocks. It is the 

 aim of the following pages to determine to .-nine extent the actual meaning 

 of i he general word " metamorphism " as applied to these rocks ; in other words, 

 to trace out as far as practicable the mineralogical changes which they have 

 undergone. 



LlTHOLI IGICAL I ►iVISK »NS. 



Lithologically the rocks of the hills are granite, ancient modified basic 

 eruptives, later eruptives, slates, quartzites and conglomerates, crystalline 

 mica-schists and mica gneisses, and ferruginous quartz. 



I leth Parallel, Vol. I Bj me ma tic Iokj 



Kins 



R. \ in Hi-.- Am I. XXXI, 1880, pp 



