74 THE MAKQUETTE IKON-IiEAlUNO DISTRICT. 



At the Repuljlic miue magnetite and niartite (hematite pseudomorphs 

 after magnetite) are frequently found near the "quartzite" of Brooks (For- 

 mation XIV). For this reason, and because the quartzite is firmly welded 

 to the ore, and l)reaks across its laminae, cutting them and sending tongues 

 into the "mixed jasper and ore," the rock is supposed to be eruptive 

 (intrusive). In one place (pp. 54-55) the author describes it as greisen. 

 Tlie quantity of magnetite present at an}^ place is thought to be dependent 

 largelv upon the abundance of eruptive dikes and their proximity to the 

 ore deposit. 



The question as to the origin of the 1)asic massive rocks of the region 

 the author answers decidedly. He finds many examples of fresh and 

 altered diabases occurring in dike-like forms, cutting the green schists near 

 Marquette, Ishpeming, and Negaunee, and traversing a "breccia or con- 

 glomerate" near Deer Lake and other j^laces. The material of the dike 

 in the Deer Lake conglomerate "is so altered that it resembles a chlorite 

 schist, and in the thin section is seen to be composed of chlorite, quartz, 

 and mica. It holds some feiTUginous masses resemljling the product of the 

 decomposition of titaniferous iron, as Avell as one or two that prolialily 

 resulted from the decomposition of olivine or brown hornblende. * * * 

 We regard tlie rock simplv as a more highlv metamorphosed condition of 

 the diorites of the region" (pp. 42-43). Other dikes were seen at the Jack- 

 son and the Washington mines, cutting the ore, and south of the Champion 

 mine, traversing the granites and gneisses. In many cases the dikes show 

 their intrusi\e character in the field, and exhibit under the microscope the 

 features usually regarded as appertaining to crystalline rocks. 



The "magnetic siliceous schist" of Brooks is learned to be composed 

 of actinolite, hornblende, magnetite, and garnet, and together with other 

 similar rocks, including Wichmann's eklogite, is believed to be eniptive. 

 The actinolite-schist south of Humboldt passes into a quartzite rock made 

 up principally of alternating layers of quartz and actinolite, and is therefore 

 sedimentary. The author thinks that the actinolite-schists were formed of 

 the detritus of the garnetiferous actinolite rocks, which are intrusive. 



From these studies it is plain that the eruptive nature of many of the 

 massive beds of "diorite" described bv earlier writers as interstratified with 



