220 THE MARQUETTE lliOX BEARING DISTRICT. 



SECTION' m.— ISOL,ATED ARKAS WITIIIX THE AL,GOXKIAX. 



In adflitinii to tlie two areas of the Basement Complex which have 

 \HH^n (liscusseil, there are isohited ])atclies of" pre-Algoiikian rocks Iving 

 entirely within the Algonkian area. Some of" these areas perhaps repre- 

 sent islands within the Algonkian sea, while others are portions of the 

 pre-Algonkian mainland that have been forced uj)ward through the over- 

 Ijnng rocks by the' forces that folded and compressed the latter. These 

 latter constitute the axes of anticlinal folds, and are naturally longer in the 

 direction of the strike of the folds, and when tlie material of the nuclei is 

 schistose the ilirectiou of tlie schistosity is usually parallel to the greater 

 diameter of the areas. They are bordered by fragmental beds belonging 

 witli the lowermost formations comprised within the folds. 



The rocks forming the greater portion of the isolated areas are gneis- 

 .soid granites and schistose greenstones that differ in no essential resj^ect 

 from the corresponding rocks of the Northern and the Southern Complex. 

 The greenstone-schists of the isolated area south of Marquette are identical 

 with the j\Iona schists.' The granites consist of the same minerals as do the 

 other granites of the Basement Complex, Isut they have become gneissic. 

 Under the microscope their constituent minerals are seen to be shattered 

 and crushed to such an extent that many sections look like those of 

 fragmental rocks. The fragments, especially those of (piartz, have been 

 rounded l)v attrition, and the feldspar has been granulated so that the 

 sections resemlile those of an arkose containing large waterworn quartz 

 o-rains. As alteration progresses tlie feldspar changes to a mosaic of seri- 

 cite, kaolin, and quartz, which often becomes so abundant as to obliterate 

 the outlines of the feldspar fragments or to wholly destroy the grains. 

 In this extreme phase of, alteration the rocks present tlie appearance of 

 sericite-schists, such as are so common in the belt of Palmer gneiss in the 

 northern border of the Southern Complex. Since many of these sericite- 

 schists occupy zones of mashing in the granites, there can be no question 

 as to their oriyiu. 



