242 i: VAN IM-i: — PRE-CAMBRIAN OF THE BLACK HILLS. 



iiti.l.i- great | In the mica-gneisses, as :i farther change, al the 



time the old feldspars were decomposing, new feldspars of a differenl 

 character were developing. Evenly granular, typical mica-schists and 

 ,,,;,■ i ;lV r thus b sen formed from coarse fi Idspar-quartz detritus. En 



these final results the positions of the feldspars are marked only by the 

 relative abundance of mica, while those of the clastic quartzes are marked 

 onlv !>v a tendency of the broken-down particles of this mineral to be larger 

 and freer from mica than the av< if the rock. 



The microscopical Btudy brings additional evidence in Bupporl of Newton's 



iclusion thai the ferruginous quartz-rocks of the Black Hills arc like 

 much of the iron-bearing formations of the Lake Superior region. 



In the development of secondary Bchistose structure, elongation of particles 

 tak.~ place perpendicular to the lines of force, while fracture takes place 

 approximately in the lines of force. 



The Black Hills furnishes an admirable instance of the existence of broad 

 belts of >latc> and schists, the structures of which arc thoroughly developed 

 and their directions entirely independenl of true beddin 



In the metamorphism of the rocks the sedimentaries and basic eruptives 

 have been affected by the same force. In this metamorphism, both in the 

 fragmental and crystalline rocks, profound mineralogical changes have 



curred. A schistose structure has been produced in both. Wethus have 

 in the Black Hill.- crystalline schists of sedimentary ami of eruptive origin. 



The Black Hill- rocks exhibit a remarkable lithological analogy to certain 

 of the iron-bearing series of the Lake Superior region, which in the past 

 have been included under the term Huronian. While this correlation is 

 not beyond doubt, there is no question that these series in common belong 

 t'> tie- AJgonkian period. 



This paper make- dm pretension i" completeness. To describe in detail 

 the many phases of the rocks of the Black Hill- ha- not even been attempted. 

 The object has been t" arrive at the structural relations ami genesis of cer- 

 tain among the rock- which occur in the pre-Cambrian area, so tar a- the 



material at haml would allow. 1 cannot close without saying a word in 



lition of the work done by Newton. My surprise is not that I find a 



- which seem to hud to conclusions differenl from his, hut that he 



i much. The an I'd in a single season i- of va-l 



1 i only had the geology of the pre-Cambrian rock- to he worked 

 that of the great fossiliferous series there found. The many 

 l>i .-il history presented by the region had to he considered. 



It was inevitable that the most of Newton'- time should b n to deter- 



mining tin I >. >im i' the periods rather than in describing in detail their 



■ on.- who first into a i egion, t he fossiliferouE 



