460 THE MARQUETTE IllON-BEAKING DISTRICT. 



SECTION III.— THE CLARKSBUKG FORMATION. 



By W. S. Bayley. 



The Clarksburg formation differs from tlie other formations of the 

 Blarquette Algonkian in that it embraces a large quantity of volcanic 

 material interbedded with sediments as regularly as the beds of an ordi- 

 nary clastic series. The series embraces surface flows of basic lava, beds 

 of tuff, conglomerates, and breccias, interleaved with well-banded layers of 

 graywacke, slate, or quartzite. These are cut by dikes and irregularly 

 shaped intrusive masses of "greenstone" similar in macroscopic appearance 

 to the "diorites" cutting the iron formation. No acid igneous rocks have 

 been discovered anywhere in the formation, either as lava flows or as 

 pebbles inclosed in the conglomerates. 



DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. 



The rocks of the formation form a belt extending westward from the 

 high bluffs north of Stoneville Station, on the Duluth, South Shore and 

 Atlantic Railway, in sec. 18, T. 47 N., R. 27 W., to the center of sec. 31, 

 T. 48 N., R. 29 W., a distance of about 12 miles (Atlas Sheet IV). Passing 

 east from its western end, near Champion the belt grows wider for several 

 miles; then it narrows, and again widens as it swings southeastward 

 toward Clarksburg, to the southeast of which village the formation 

 reaches its greatest width of about 1^ miles. From this point the belt 

 swings to the east again, and becomes gradually narrower until it disap- 

 pears in sec. 18. 



It is noticeable that where the belt has its maximum width the under- 

 lying formations swing southward, and that as the belt narrows to the east 

 and west of Clarksburg they reassume their normal courses. 



The formation is a local one in the sense that it occurs on one side only 

 of the syncline in the Upper Marquette series. The central vent from which 

 most of the lavas and tuffs were erupted is thought to have been in the 

 widest portion of the belt, a little to the southeast of Clarksburg. Here 

 the tuffs and sedimentary rocks are rare and the intrusive boss-like knobs 

 of "greenstone" are most numerous. But east and west of this place other 



