23-1 i i: \ : W iil-i ■■— l'i:i.-< • \ M I : I : I w OF THE BLACK HILLS. 



entire area in which the directi >f pressure is constant, and this oftentimes 



is |arg( The folds formed at the same time cause the bedding to have wide 



riations in <li|>, unless the squeezing be carried bo far as to make the sides 

 of the folds parallel. In thi> extreme case the cleavage-foliation and strati- 

 fication would ordinarily be bul slightly or nol al all discordant. 



It is evident, in the Black Hills pre-Carabrian area, thai the crusl of the 

 earth has probably uol been compressed to Buch an extent as \\ « >n It 1 have 

 been the case h :i< 1 Beveral or many close parallel folds been formed bo that 

 the structure would represent bedding. Such a process implies great crustal 

 shortening. Gentle folding, and therefore a small percentage of diminution 

 in area, is often sufficient to thoroughly develop transverse slaty cleavaj 

 Mistaking cleavage-foliation for Bedimentation in areas of widely extended 

 parallel structure, where the theory of repeated folds is resorted to, would 

 lead, in most cases, to an over estimate of the amounl of the shortening of the 

 crusl of the earth in the supposed folding proi 



( JORREI \ I [ON. 



The slates and schists of the Black Hill-— that is, the great mass of the 

 pre < lambrian rocks ofthaf area, have been Been to be of clastic origin. This 

 suggests the question, <1" these rocks belong to the most ancient known com- 

 plex of the earth's crust ? In certain localities in the Lake Superior country 

 there are extensive areas of an intricate complex of granite, gneiss, :ui<l com- 

 pletely crystalline Bchists,* older than any rocks which have been shown to 

 be clastic and separated from them by a great unconformity. The larger 

 part of the pre-Carabrian areas of the far west are also of like character. 

 IV - [rving and Bonney \ have been inclined to believe that the condi- 



tions which produced this wholly crystalline complex havrn.it been repeated 

 in the world's history. It was formerly assumed that schistose structure 



a proof of sedimentary origin. It i- now generally conceded that this 

 structure is often found in eruptive rocks. This being the case, our own 

 Btudies have wholly failed to find anywhere in the northwestern country any 

 positive proof of clastic origin for any of these fundamental rocks, although 

 I incline to the belief that certain of them are profoundly modified IV. 

 mentals. The old ruck- of the Highlands of Scotland belong t" the class 

 under consideration. An exhaustive stud} of this region has been recently 

 made by the Geological Survey of Great Britain. Dr. Archibald Geikie, 

 tic 1 1 era! ofthat Survey, in a recent paper, concludes not only that 



there is no trace al present of clastic character in any of these rocks, but 



■■ii. in Formations, R L) Irving Seventh \nn 

 til of i in- ■ of London, I ' • Bonn< 



lini;-, pp II" 112, 



