670 W. O. HOTCHKISS THE LAKE SUPERIOR GEOSYNCLINE 



F0R^fATI0X^-I?s'Cm3>ED I2f THIS-SY>vGLtXE 



The oldest rocks in this svncline are the Iveewatin greenstones and 

 Laurentian granites. These had undergone much deformation and 

 metamorphism and were base-levelled before Hnronian time. 



On this plain were deposited the Hnrpnian sediments of the Mesabi 

 and Gogebic districts. The rocks of these two districts are so mnch alike 

 that only those in the Gogebic are described. On the Archean basement 

 is first a Lower Hnronian dolomite, present in the Gogebic but lacking 

 in the Mesabi. Eemnants only are left of this. Xext follows the Palms 

 quartz slate, which has a fairly uniform thickness of 500 feet over most 

 of the range, but thickens rapidly at the east end to about 1,000 feet. 

 This is a greenish gray, unoxidized formation. 



The Palms is succeeded by the Ironwood formation. This is about 

 500 feet thick at the west and thickens gradually to about 1,150 feet at 

 the east end of the range. The same beds which make up the 500-foot 

 section on the west are 900 feet thick in the east. The top of this forma- 

 tion has certainly been eroded, but the differences in thickness are 

 probably due more to original deposition than to erosion. 



The Ironwood is succeeded by the Tyler formation — a great unoxi- 

 dized, greenish gray slate and graywacke series with much iron carbonate 

 near the base. It reaches a thickness of 2% miles a short distance west 

 of the Michigan- Wisconsin line and tapers to nothing 20 miles to the 

 east and west. Here again the top of the formation is eroded, but how 

 much the varvino- thickness is due to erosion and how much to orio'inal 

 deposition can not be stated with certainty. Again both factors probably 

 enter into the complete story. 



The Tyler is followed by the Lower Keweenawan sandstone or quartzite 

 and conglomerate. This formation is poorly exposed and probably is not 

 always present. Its thickness is less than 100 feet, as a rule. It is 

 chiefly a brown to yellowish oxidized formation. When pebbles are 

 present, they are predominantly of quartz porphyry. 



Then follow the Middle Keweenawan basic flows and intrusives, with 

 occasional quartz porphyry flows and intrusives, and acid conglomerate 

 and quartz sand beds. This series varies in tliickness from 3 miles to 

 6 miles. 



"DiRECTioxs OF Slopes ox w^hich Shdimkxts axd Flows were 



DEPOSITED 



The direction of the original slope of the Archean baselevel is not 

 known. 



