GEOLOGICAL EXPLOKATIONS AND LITERATURE— ISSO. G9 



of the i;i8pers given. The ;iuth(ir had not visiteil the Min-([uette district, 

 so he must luive obtained his n.otions ot" tlie rehitions of the jasper.s from 

 the hterature on tlie area. The jasjjers Jiad freciucmtly been described 

 as interstratified with chlorite-sehist, etc., but nowhere had it been shown 

 that tlie two rocks grade into each other. Moreover, the ]\larquette ore 

 does not usuulh- contain from 1 to 2 per cent of manganese. 



isso. 



Brooks, Thomas Benton, The geology of tlie Menominee iron region (east of 

 center of Range 17 E.), Oconto County, Wisconsin. Geology of Wisconsin, 1873-1879, 

 Vol. Ill, pub. in 1880, Part VII, pages 429-599. 



The Wisconsin reports, ahhough they are devoted mainly to the discus- 

 sion of Wisconsin problems, contain numerous references to the geology 

 of Michigan. In Brooks's report on the geology of tlie Menominee iron 

 range are several references to the Marquette rocks. There is, besides, a 

 new table of the formations in the Marc^uette Huronian. The principal 

 difference between this table and. that published in the j\Iichigan report is 

 the addition of Formation XX, which embraces tlie "granites" southwest 

 of Lake Michigamme, thought to be younger tlian the mica-schists of the 

 vicinity. The references to the Marquette series are mainly with respect 

 to their correlation with the Menominee series. 



As an appendix to this report (pp. (!61-GG3) Bi'ooks gives a brief sketch 

 of the Laurentian rocks in Michig-an. This series includes inica-g-neisses, 

 hornblende-gneisses, hornblende-schist, chloritic gneisses, chloritic schists, 

 often derived from the mica-gneisses and hornblende-gneisses, and granites. 

 The latter rocks are the massive varieties of g-neiss "in which all interior 

 evidence of bedding is obliterated by inetamorphic action." All the o-ran- 

 ites of tlie Laurentian, with the exception of the dike granites "and certain 

 great irregular red masses," are thought by the author to be altered sedi- 

 mentary rocks. 



The Laurentian rocks are cut in all directions by dike-like masses of 

 granite and greenstone, of which the latter are "far thicker, more regular 

 and persistent than those of the granite." 



