PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 565 



beds, Upper cherty beds, Lower slaty beds, and Lower cherty beds. 



Broderick^ interprets certain negative magnetic lines of the 

 Duluth gabbro as due to a certain angle of inclination of the mag- 

 netic formations with the horizontal. 



According to Cayeux^ traces of crinoids are found in the iron 

 formations of the Gogebic, Mesabi and Menominee ranges. They 

 consist of circular, quadrilateral, and hemispherical bodies larger 

 than the oolites and of polygonal cells whose walls are composed of 

 iron. The cells occur with and without alignment. 



Grout^ finds that siliceous pegmatites formed on all sides of the 

 basic Duluth gabbro, but that distinct dikes occur only outside 

 the contact. He infers that the two magmas separated in the 

 liquid state. 



Grouf presents interesting petrographic descriptions of the 

 Biwabik iron formation of the eastern part of the Mesabi district. 

 He concludes that originally the iron formation was a shallow 

 water deposit formed mainly by organic processes. 



Grout^ proposes the name lopolith for intrusions like the 

 Duluth gabbro whose floors and roof sag downward toward the 

 middle. Evidence is introduced for concluding that the main 

 mass of this intrusion was along a plane of unconformity. Sug- 

 gestions are made that the method of intrusion of the lopolith is 

 different from that of a laccolith. 



Grout believes, as indicated by his diagrams, that the dip of the 

 pre-Cambrian beds around the western rim of Lake Superior is 

 due to settling rather than compression. It would be interesting 

 to find out whether the nature of the fracture and flow cleavage 

 structures in these formations checks with this view. 



' T. M. Broderick, "Some Features of Magnetic Surveys of the Magnetic Deposits 

 of the Duluth Gabbro," Econ. GeoL, Vol. XIII (1918), pp. 35-49. 



^ L. Cayeux, "Existence de restes organique dans le roche ferruginenses associees 

 aux minerals de fer huroniens des Etats-Unis," Acad. Sci. Paris Compt. rend., Vol. 

 153, PP- 910-12. 



3 F. F. Grout, "The Pegmatites of the Duluth Gabbro," Econ. GeoL, Vol. XIII 

 (1918), pp. 185-97. 



4 F. F. Grout, "The Nature and Origin of the Biwabik Iron Bearing Formation of 

 the Mesabi Range, Minnesota," Econ. GeoL, Vol. XIV, (1919), pp. 452-64. 



s F. F. Grout, "Lopolith, an Igneous Form Exemplified by the Duluth Gabbro," 

 Am. Jour, of Science, Vol. CXCVI (1918), pp. 516-22. 



