(lEOLOGICAL EXPLORATIONS AND LITERATUKE— 1895. ]45 



of unfOiift)niiity. The iron-bearing rocks on both sides of the qnartzite 

 are of the same age, and, indeed, are portions of the same formation; 

 conseqnentl)^ there are not two ore horizons in tlie Repnbhc area, as 

 Wadsworth snpposed. 



The Repnbhc structure is described as a s^-nchne some 7 miles hjng'. 

 Its axis runs about northwest, and is nearly horizontal, except at its south- 

 eastern end, south of Smith Bay, Avhere it dips about 45° to the nortliwest. 

 The rocks are thus exposed in a horseshoe-shaped curve. They have been 

 squeezed nearly into parallelism on the two sides of the axial plane, the 

 Lower Marquette rocks dipping a little more steeply than those belonging 

 in the U})per ]\Iar(|uette series. The radius of tlu; ciu've at the toe of the 

 horseshoe, measui-ed from the base of the upper quartzite, can be very 

 little greater than the thickness of that formation. The pr-essure caused 

 by this sharp folding has not only crushed some jtortions of the more 

 brittle rocks affected by it, but has also ])roduced three synclines and two 

 anticlines subordinate to the main syncline. 



1S95. 

 WiNCHELL, N. H. The origin of the Archeau greeustones. Twenty-third 

 Ann. Rept. Geol. and Nat. Hist. Surv. of Minnesota, 1895, pages 4-35. 



N. H. Winchell criticises Williams's work in the greenstone-schist areas 

 of the Marquette and Menominee districts, but adds nothing to our knowl- 

 edge concerning them. The author seems to believe that Williams had 

 concluded that the greenstone-schists of the Marquette district are mainly 

 squeezed irruptive rocks, whereas the strong point of his paper is the doc- 

 trine that they were originally basic tuffs and surface lava flows. After 

 discussing the j^roblem somewhat at length, Winchell reaches very nearly 

 the same conclusion as does Williams, i. e., he concludes that the greenstone- 

 schists are mainly altered tutfs of basic rocks. He does not believe, how- 

 ever, that they reached their present condition through the action of 

 d}'namic metamorj)hism, but the processes by which they liave becqme 

 schists are not clearly set forth. 



The author furthermore ascribes to the schists a definite horizon at 

 the close of the Archean, and places under them an iron-bearing- formation, 

 MON xxviii 10 



