124 THE VERMILION IRON-BEARING DISTRICT. 



to be Archean and to underlie unconformably the Beaver Bay diabase, are 

 believed to represent segregation phases in the main gabbro flow, and to 

 be the same as anorthosite masses in the gabbro proper to the west. 



The Beaver Bay diabase is believed to represent the upper portion of 

 the Great gabbro flow, and to be due to the first and greatest movement of 

 the gabbro toward Lake Superior. The Logan sills belong to this part 

 of the gabbro flo'vi^. 



The Manitou division of the Keweenawan includes the surface flows, 

 sills, and dikes which accompanied and followed the Puckwunge conglom- 

 erate. These eruptives, with the elastics associated with them, do not have 

 a thickness in Minnesota of more than 1,000 feet. These lava sheets extend 

 along the shore of Lake Superior from near Baptism River to near Grand 

 Marais, except where replaced at intervals by the Beaver Bay diabase or 

 some of the intersheeted fragmentals. They occur also in the neighborhood 

 of Grand Portage Bay, but their extent here is not definitely known. 



General. — The most important petrologic conclusions reached from the 

 examination of the Minnesota crystalline rocks are three in number: 



1. All the granites of the Archean can be explained on the assumption 

 that they are intrusives representing the metamorphosed conditions ot 

 clastic rocks adjacent to the observed intrusions, rendered plastic by the 

 force of dynamic metamorphism accompanied by moisture. 



2. The Keweenawan gaboro and its derivatives are derived from the 

 metamorphism and refusion of the Archean greenstones and their attendants. 



Comment. — The two main petrologic conclusions announced by Pro- 

 fessor Winchell as the most important results of his final petrologic work, 

 summarized in the closing general paragraph, would be dissented from by 

 most of the other geologists who have worked in this area." 



The Cambrian age of the Animikie strata has long been maintained by 

 Professor Winchell, and above are summarized his arguments in support of 

 this position. The first argument, that the Animikie grades into the Upper 

 Cambrian rocks, is not in accord with the observations of most of the 

 geologists above referred to. The second argument, based on the similarity 

 of the unaltered greensand in the Mesabi district to that in the Cambrian 

 of the eastern United States, loses weight when we consider the fact that 

 the similarity is not great, the differences being many and significant; 



"Some of these geologists are: R. D. Irving, C. R. Van Hise, J. Morgan Clements, W. S. Bayley, 

 U. S. Grant, J. E. Spurr, A. H. Elftman, C. K. Leith. 



