140 TOE MARQUETTE IRON-BEAKING DISTRICT. 



calls lieaconito, and also analyses of a cliloritoid, an ainpliibole, and a 

 clay. A list of minerals occnrrint;- in tlie Upper Peninsula concludes the 

 cliapter. 



Lane, A. 0. Microscoi)ic cliaracters of rocks and minerals of Michigan. 

 Kept, of State Board of Geol. Surv. for 1891-92, Lansing, 1893, pages 176-183. 



In an a])j)endix to Wadsworth's report Lane describes very brieil)' the 

 microscopic characters of some of the most interesting- rocks of the Marquette 

 district. 



Among these the class of the quartz-diabases is mentioned as well 

 characterized by the presence of quartz, sometimes, indeed, only in small 

 quantity. The rocks are the youngest eruptives of the district. The}' 

 occu])A' well-defined dike tissures, and have altered the slates adjacent to 

 them. Their quartzose component is iisually in the interstices between the 

 diabasic constituents, and is frecjuently intergrown with plagioclase. It is 

 regarded as original. The rocks are present in some of the n^iines, and in 

 these cases they seem to have aided in guiding the aqueous currents active 

 in producing the ore, but they are different from the miners' "diorites," and 

 are younger than these. 



The amphibolites and hornblende-schists of the author include the 

 "diorites" referred to. They are all secondary rocks, having been derived, 

 in all prol)al)ilitA', from diabases. In the alteration of tlie latter rocks into 

 the former ones iron o.xides were removed and were condensed into ore 

 bodies. Transitions from chlorite-schists, which are often but marginal 

 forms of the amphibolites, into ore bodies can he seen at the Chamjnon 

 mine, the magnetite replacing the eruptive rock in j)laces. The author 

 ascribes to some of the ores of the district this origin, but excludes the soft 

 ores from the category. 



Patton, H. B. Mi('roscopic study of some .Michigan rocks. Ibid., i)ages 

 184-180. 



In another appendix Patton deals with the microscopic features of 

 some of the rocks referred to in Wadsworth's report. Aplitic diorites are 

 mentioned as occurring in the SE. ^ sec. 14, T. 48 N., R. 25 W., and on the 

 Middle Picnic Island. 



