Distribution \m> Structure of the Rocks. 



As shown by the accompanying map (fig. I I, copied from Newton's report, 

 the pre-Cambrian rocks are divided into an eastern or slate and a western or 

 schist area. Within the schist area arc located several detached masses of 



inite, one of considerable size, the highest peak of which is the culminating 

 point of the hills. The slates and schists arc described by Newton as verti- 

 cal and as having in general a strike approximately north and Bouth, or a 

 little wesl of north and south of east, with wide local variation. Although 

 Newton and Jenney looked for proof of discordance between their slate and 

 schisl series, they found no positive evidence of it, although at one obscure 

 locality Jenney thought he saw such indications.* No evidence that the 

 Bchisl or slate is folded was found, and it was thought their combined thick- 

 ness is very great, being represented by the surface exposure of the pre- 

 Cambrian core in an cast and west direction. The following paragraph is 

 from Newton : 



lination brought to light no evidence of the duplication of any part- of 

 the Archean rock Bystem. If the 3lates or the schists were folded upon themselves 

 and afterwards worn away, so a- to leave two or more parallel outcrops of the same 

 beds, tic folding must bave 1 n confined to the homogeneous soft beds : and the pre- 

 sumption is Unit no such folding took place within the area exposed in the hills. 

 The \\ bole system of vertical beds, with a width of about twenty-live miles, i- believed 

 to retain it- original .'-elation of parts. It. has not. of course, it- orignal position, for 

 the same great process of change which has produced it- metamorphic structure has 

 turned it bodily on edge and either broken away or eroded away in upward continu- 

 ation : hut it i< probable that the system prevents tic clays and -hah- ami sandstones 

 from which it wa- produced by metamorphism in the same order in which they were 

 originally depo 



The enormous thickness of sediments which this explanation requires was 

 realized bj Newton and ( rilbert, and was evidently regarded as a fact against 

 it- corrects -- No subsequent writer has attempted to re-examine (he evi- 

 dence upon which this great thickness for the pre-Cambrian rocks is based. 

 My study of the -late area agrees with Newton's observations that the 

 rock series ha- a cleavage which is practically vertical. Howcver.it was 

 ascertained that this parting i- in the nature of Blaty cleavage rather than 

 ' trui' bedding. The fact that there are partings in two directions in certain 

 localities ha- been noted by both Crosby and Carpenter, bul particularly 

 the latter, who interpreted these to mean that the rocks had been subjected 



to pressure in two directions : and in some places this explanation is the true 



one. 



Dakota, p 





