34 THE MARQUETTE lEON-BEAKING DISTRICT. 



Reference has already been made t<i the discovery by the authors 

 of great numbers of greenstone dikes in the Azoic schists, and of knobs of 

 greenstone scattered among those of the ores. They iind great difficulty 

 in distinguishing between the greenstones of igneous and those of meta- 

 morphic origin. " The passage of one into the other, especially in the 

 southern j)ortion of the district, seems in many cases to be gradual, and 

 their general appearance and structure is S(j much alike that it is often 

 difficult to say where one begins and the other ends." Evidently the 

 authors regard many of these greenstones as forming portions of volcanic 

 flows. They class them with the diorites. 



The unconformity at Granite Point between the granite and the 

 sandstone, and that at the Carp River between the latter rock and vertical 

 quartzite, were observed and correctly interpreted. 



From 1850 to the appearance of Brooks's report in the Geology of 

 Michigan in 1873 A-ery little additional information Avas published concern- 

 ing the relation of ore deposits to their associated rocks in the Marquette 

 district. A number of short papers appeared in this interval, but they 

 treated only of small points in the geology of the region, and none dealt 

 with the relations of the rocks to one another. 



BiGSBY, John J. On the physical geography, geology, and commercial 

 resources of Lake Superior. Edinburgh oSTew Phllos. Jouru., No. 105, July, 1852, 

 pages 55-62. 



Bigsby gives a general account of the geology of the Lake Superior 

 region, which is based largely on the reports of the earlier investigators. 

 It contains nothing that had not already been conunented upon by others. 



1854. 



Whitney, J. D. The metallic wealth of the United States. Thlladelphia, 

 185-1. H. R. Schoolcraft. 510 pages. 



Whitncv, in 18,54, published a volume containing a few references to 

 the iron ores of Michigan, repeating the statements made in Foster and 

 Whitney's report on the Iron Region. The ores are described as occurring 



