THE BASIC MASSIVE ROCKS OF THE LAKE SUPERIOR 



REGION. 



III. THE GREAT GABBRO MASS OF NORTH-EASTERN MINNESOTA. 1 



A. Introduction. 



As has already been stated in an earlier paper, 2 the writer 

 purposes, as time and opportunity permit, to discuss the petro- 

 graphical and stratigraphical relationships of the basic rocks that 

 constitute such an important element in the geology of the coun- 

 try bordering Lake Superior. In the series of papers, of which 

 this is the first, the petrographical characteristics of the various 

 types of these rocks will be described, and the views held by 

 previous workers with respect to their geological relationships 

 will be outlined. Thus, it is hoped, a foundation will be laid for 

 a new and more thorough investigation of the field relations of 

 these rocks than has heretofore been possible. As the case now 

 stands, several of the geologists who have investigated the 

 eruptive rocks of this region have erred in confusing types of 

 entirely different origins, and have thereby introduced into the 

 literature errors of observation that have rendered a clear under- 

 standing of the Lake Superior geology almost impossible. 



When practicable the laboratory and field study of rocks 

 should proceed together, each aiding the other in solving the 

 knotty problems that so often arise in their progress. The 

 laboratory study of the eruptives in the region under considera- 

 tion has been almost entirely neglected, and consequently the 

 field problems arising in connection with them have largely 

 remained unsolved. When the peculiarities of these rocks — 

 their composition and structure — become known, much light 

 will be thrown upon their nature, and it will then be time to 

 again review their field relations, when it is believed that many 



'This Journal, Vol. I., pp. 433 and 587. 

 2 This Journal, Vol. I., p. 435. 



