THE PEKIDOTITE. 183 



SE. corner of sec. 18, T. 48 N., R. 28 W. It is an aggreg-ate of plagloclase, 

 orthoclase, quartz, and biotite. All the components are much altered. 

 The biotite is chloritized, the orthoclase kaolinized, and the plagioclase 

 saussuritized. Plagioclase and quai'tz compose the greater portion of the 

 rock. The former is in little grains with irregular outlines that exhibit a 

 tendency to become quadrangular, and the latter in grains between tlie 

 plagioclases. In strui-ture the rock is j)anidioniorphic. 



The quartz-porphyry dikes have already been studied by Williams. 

 The rocks are light-colored. They sometimes still have their original char- 

 acters sufficient]}' well preserved to exhibit the pbrphyritic structure. In 

 other and more numerous instances the rocks are schists. Their })or})h\-- 

 ritic orthoclases are broken and their fragments displaced, their porphvritic 

 quartzes are granulated, either entirely or only peripherally, while the 

 quartz-orthoclase mosaic that originally constituted their groundraass is now 

 a schistose aggregate of quartz and sericite. In not a few of the pt)rj)hyries 

 plagiocliise accompanies the orthoclase as phenocrysts, and chlorite, filled 

 with tiny brown rutile crystals, is distributed through the groundniass as 

 laminse apparently replacing an original Ijiotite. 



THE PEEIDOTITE. 



The serpentine and peridotite form high, ragged bluffs that are notice- 

 able for their dark color and jagged contours. One of these bluffs forms 

 Presque Isle, on the shore of Lake Superior, about 2i miles north of 

 Marquette. The principal occurrences of the two rocks are, howe\'er, 

 northwest of Ishpeming, in the area of the Kitchi schists. 



TIIH I'RESQUE ISI.E AREA. 



The rocks of the Presque Isle area (Atlas Sheet XXXVIII) are so well 

 known, thanks to Dr. Wadsworth,^ that we need give them little attention. 

 They were all originally peridotites, but they have undergone alterations 

 due to weathering until they are now largely serpentines and dolomites. 

 Among the freshest phases of the rock Iherzolites, picrites, and wehrlites 

 have been distinguished.^ 



' Lithological Studies, by M. E. Wadaworth, 1884, p. 136. 



2 Report of the State Geologist for 1890-91, by M. E. Wadswortli: Kept. State Board Geol. Surv. 

 Michigan 1893, pp. 134-138. 



