568 EDWARD STEIDTMANN 



Leith^ suggests that the unconformity at the base of the Cam- 

 brian was developed by a process of cut and fill, and that the 

 common occurrence of late pre-Cambrian terrestrial sediments 

 is more than a coincidence, but is related to the development of 

 the basal Paleozoic unconformity. 



NebeP presents a petrographic study of certain basal portions 

 of the Duluth gabbro and its contact effects. 



Powers^ reports that drilling in the east-central portion of 

 Kansas has shown the existence of pre-Cambrian granite of con- 

 siderable relief occurring along a north-south line. 



Wolff'' reports that the average thickness of the Mesabi iron 

 formation is six hundred and twenty feet, and that it consists of 

 four divisions which from the top down are as follows: Upper 

 slaty horizon, Upper cherty horizon, Lower slaty horizon, and 

 Lower cherty horizon. The ores occur chiefly in the two cherty 

 horizons and in the Lower slates. Marked differences exist 

 between the ores of the various horizons. 



'C. K. Leith, "Relations of the Plane of Unconformity at the Base of the 

 Cambrian to Terrestrial Deposition in Late Pre-Cambrian Time," Congres Geologique 

 International, XII. Session Canada, pp. 333-37. 



^ M. L. Nebel, "The Basal Phases of the Duluth Gabbro Near Gabamichigami 

 Lake, Minnesota, and Its Contact Effects," Econ. GeoL, Vol. XIV (1919), pp. 367-402. 



•' Sidney Powers, "Granite in Kansas," Am. Jour, of Science (4th ser.), Vol. XLIV 

 (1917), pp. 146-50, I fig- 



4 J. F. Wolff, "Recent Geologic Developments on the Mesabi Range, Minnesota," 

 Am. Inst. Min. Eng. Bull. No. 118 (1916), pp. 1763-87, 14 figs. 



[To be continued] 



