120 Btdleti7i of Laboratories of Denis on University. [Voi. xii 



George Steiger is shown in column VIII of the table of analyses, 

 on pages 1 12-1 13. 



The South Mountain area is shown by Williams^ and Bas- 

 com^ to consist of the acid volcanic rhyolite and the basic 

 types, diabase and basalt, the latter yielding on alteration the 

 greenstones of the region. Bascom^ further describes the basic 

 types of this region as holocrystalline, effusive, plagioclase- 

 augite rocks, with or without olivine, the essential characteris- 

 tics of the diabase group. 



After establishing the igneous origin of the greenstones of 

 the Menominee and Marquette districts of Michigan, Williams^ 

 shows the different rock types to have been olivine-gabbro, 

 gabbro, diabase, diabase-porphyry, glassy diabase and mela- 

 phyre, and tuffs, with the two districts limited on their north 

 and south sides by an acid series consisting of granite, granite- 

 porphyry, and quartz-porphyry. 



The original mineral constituents of these rocks are de- 

 scribed by Williams^ as labradorite, quartz, biotite, hornblende, 

 diallage, augite, olivine, zircon, apatite, sphene, ilmenite, and 

 magnetite. The secondary minerals produced by metamorph- 

 ism and weathering are albite, saussurite, zoisite, quartz, horn- 

 blende, epidote, chlorite, biotite, talc, serpentine, carbonates, 

 iron oxides and pyrite.® 



Van Hise and Clements regard the greenstone schists of the 

 Crystal Falls iron-bearing district as altered diabase, diabase- 

 porphyry, and gabbro.^ Clements^ has shown the derivation of 

 the greenstones of the Hemlock formation to be from original 

 basaltic and andesitic rocks. 



1 Op. cit. 

 ' Op. cit. 

 ^ Ibid., p. 69. 



* Bull. U. S. Geol. Survey, No. 62, pp. 197-199. 

 ' Ibid., pp. 199, 200. 



• Ibid,, pp. 213, 214. 



' Mono. U. S. Geol. Survey, vol. xxxvi, pp. 484-4S6 ; ibid., vol. .\xviii, 

 pp. 203-20S. 



* Ibid., vol. xxxvi, pp. 95-148. 



