66 BULLETIN OF THE 



238, 239, 240, 241, 242, 243, 244, 245, 24G). Associated with and 

 comprising probably part of the same formation is a greenish and grayish 

 limestone (247, 248, 249, 250, 251). In one place much chrysotile was 

 seen, which had formerly been regarded as asbestus (252). 



No. 235 is a beautiful green serpentine, giving a colorless section 

 holding magnetite. Under the microscope this shows a most beautiful 

 fibrous aggregate polarization. Other sections (241, 242, 243, 245) 

 show a more coarsely fibrous aggregate polarization, and are composed 

 of serpentine and magnetite. The magnetite is seen here to be arranged 

 in the same way that it was in the more decomposed peridotite of Presque 

 Isle, forming a network corresponding to the outlines of the grains and 

 fissui-es in the olivine, as well as occasionally a network in the altered 

 olivine itself. Although we find no trace of either olivine or enstatite in 

 this serpentine, this structure gives a strong probability that this rock 

 was originally in nature and origin a peridotite. Every microscopic 

 character in this serpentine indicates that it is formed by direct altera- 

 tion in situ from another rock, and, so far as we can now tell, that was 

 an olivine one. Whether this is of the same age and origin as that at 

 Presque Isle or not can only be determined, if at all, by studying its 

 relations to its associated rocks. 



General Discussion and Besults. 



The historical part of this paper we have endeavored to bring down 

 to the date of its completion. As the historical portions of both the 

 Iron and Copper districts were, in the main, written in 1879, the lat- 

 ter material has been added as best it could be done. On March 6, 

 1880, through the courtesy of Prof. T. C. Chamberlain, Chief Geolo- 

 gist of the Wisconsin Survey, we received an advance copy of that por- 

 tion of the third volume of his survey publications which gives Dr. 

 Arthur Wichmann's microscopic analyses of some of the rocks in the 

 Iron district and Appendices A and B, or from pages 600 to 663. 

 This detached portion has been referred to repeatedly in the preceding 

 pages, but no connected account was or need be given of it. On April 8 

 the complete volume was received, but as the preceding portion of our 

 work was, with the exception of a few pages, already prepared for the 

 press, and in part struck off, it became necessary to incorporate all 

 mention of it in this portion of the paper at this date (April 10, 

 1880). 



The observations and figures given in the preceding text show conclu- 

 sively that the statements of Messrs. Dana, Kimball, Hunt, Brooks, and 



