482 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



ARKANSAS. 



It appears that Dr. David Dale Owen regarded the crystalline rocks 

 of Arkansas as eruptive, stating that the sedimentary rocks have been 

 shattered, uptilted, and broken by them. He also gives localities in 

 which they overlie the slates. His evidence, what there is of it, would 

 indicate the correctness of his views, but it is far from being as com- 

 plete as would be desirable. 



MISSOURI. 



There is no published evidence showing that the crystalline rocks of 

 Missouri proved to be older than the Silurian are of different ages, or 

 that most of them are not eruptive. They have been assigned to the 

 Laurentian and Huronian, but solely from their lithological characters. 



MICHIGAN AND WISCONSIN. 



A short time ago a discussion of the various theories regarding the 

 crystalline rocks of the region adjacent to Lake Superior, and especially 

 its southern shore, was published by one of the authors of the present 

 paper.* In that work the evidence by which the theories in question 

 have been sustained was pretty thoroughly examined, and it now re- 

 mains, therefore, only to add that which is necessary in order to bring 

 the discussion down to the present date. Some little repetition and 

 recapitulation will, however, be advisable, in order that the reader of 

 this paper may have a clearer idea of the points here discussed. 



The copper-bearing rocks of Lake Superior have been considered by 

 some geologists as a distinct formation, older than the Potsdam sand- 

 stone adjacent to which it lies. The principal evidence in support of 

 this opinion was supposed to have been obtained at the falls of a branch 

 of Torch River, known by the name of Douglass Houghton. Here the 



* Notes on the Geology of the Iron and Copper Districts of Lake Superior. By 

 M. E. Wadsworth. This Bulletin, ante [Vol. VII., or Geol. Series, Vol. I.], 

 pp. 1-157. 



