70 THE MARQUETTE lEON-BEAETNG DISTRICT. 



WiCHMANN, Arthtik. Micfoscopical observ^ations of the iron-bearing (Ilnro- 

 nian) rocks from the region south of Lake Superior. Geology of Wisconsin, 1S73-79, 

 Vol. Ill, 1880, pages GOO-656. 



A series of about 500 tliin sections of rocks from the Penokee, the 

 Meuomiuee, and the Mar([uette iron districts ^yas submitted by Brooks to 

 Arthur Wichmann for microscopical study. Wichmann's report aimounces 

 the resuhs of the studv and the conchisious reached Ijy liim concerning the 

 origin of some of the rocks investigated — conchisions that have ah-eady 

 been referred to in some instances. The rocks are divided into hthokigical 

 groups as follows: nonfragmental rocks, including simple rocks, massive 

 rocks, and schistose rocks; aiid fragmental rocks, including clay-slates, 

 sandstone, and chert-breccia, of which the latter occurs only in the Ponokee 

 district. Among the simple rocks are found limestone, dolomite, quartzite, 

 magnetite-schist, jasper-schists, chert schists, hematite-schist, and serpentine; 

 among the massive ones, granite, syenite, diorite, and diabase; and among 

 the schistose ones, gneiss, mica-schist, hornblende-schist, chlorite-schist, 

 augite-schist, talc-schist, sericite-schist, and eklogite. The serpentine is an 

 altered olivine rock. Diabases are abundant in the Marquette district, and 

 are eruptive in origin. By addition of hornblende to the diabases, diorites 

 result; lience the diorites are also eruptive. The talc-schists described by 

 Brooks in Bed XIY are sericite-schists of sedimentary origin. 



Hunt, T. Sterry. Letters and notes on tbe iron-bearing and associated 

 rocks of the Marquette region, and comparisons with tbe Archeau of Canada and 

 of the eastern United States. Appendix A, Geology of Wisconsin, 1S73-79, Vol. Ill, 

 1880, pages 657-CGO. 



In the same volume in which Brooks's report is ])ublishcd are several 

 letters and notes by Hunt on tlie iron-bearing and associated rocks of 

 the Marquette district. One of these letters is from Hunt to Alexander 

 Winchell, and is dated November 26, 1809. In this Hunt mentions the 

 difficulty of assigning names to the greenstone-schists of northern ilichi- 

 gan, which include altered varieties of many distinct rocks. In a letter to 

 Brooks dated February 22, 1871, an attempt is made to correlate various 

 members of the ]\Iarquette series with the Huronian and Montalban rocks 

 of New England and the Adirondacks. In a third letter Hunt expresses 



