AECHEAN IN FELOH MOUNTAIN DISTRICT. 



389 



The following analyses give the chemical composition of these granites 

 Analyses of granites. 



[By Dr. H. S. Stokes, U. S. Geol. Survey.] 



1 Ba, Sr, Li, CI, S, SOa were not looked for. - Water not determined. ^ Includes PiOr,. 



No. 1. Specimen 34677, Lake Superior Division, U. S. Geol. Surv., 1,935 N., 1,040 W., sec, 2, T. 41 N., 

 R. 30 W., Upper Peninsula of Michigan. 



No. 2. Specimen 34828, Lake Superior Division, U. S. Geol. Surv., 300 N., 1,8.50 W., sec. 36, T. 42 N., 

 R. 29 W., Upper Peninsula of Michigan. 



No. 3. Specimen 36081, Lake Superior Division, U. S. Geol. Surv., 15 N., 1,025 W., sec. 31, T. 42 N. 

 R. 28 W., Upper Peninsula of Michigan. 



No. 1 , which is rather low in alumina, iron, and lime, is a granitic gneiss 

 in which the abundant secondary mica, which has grown in long curving 

 plates in nearly parallel zones of granulation, is wholly muscovite. Nos. 2 

 and '6 are fine- and coarse-grained pink granites, which show comparatively 

 little crushing and development of secondary minerals in thin section. 



The rocks of this division therefore have the chemical composition as 

 well as the physical and petrographical characters of igneous granites. 

 The positive proof of igneous origin, hoA^ever — actual injection into older 

 rocks — we have not found. Irruptive contacts may possibly exist, and 

 may have escaped us, since neither the Archean as a whole nor its internal 

 relations were the objects of especially rigid scrutiny. Igneous granites 

 of Algonkian or later age ought to be found within the Archean areas, for 

 several granite dikes are known to penetrate various members of this over- 

 lying series. Whether the known granites within the Archean are really 

 lower-lying and larger masses witli which such dikes are genetically con- 



