504 THE MARQUETTE IRONP.EARIXG DISTRICT. 



variouslv orientated, several together often forming small prismatic crystals 

 idiomorpliic in the prismatic zone, and the whole forming a very irregular 

 area. Nearly all of the areas are cellular. Their component -parts are 

 filled \\ith inclusions of the quartz, feldspar, epidote, etc., that make up 

 the body of the rock. The hornblende in these instances is certainly 

 secondary, and was jirobably the last of the principal rock constituents 

 to form. Fig. 4 of PI. XXXII illustrates the structure of one of the clus- 

 ters in a greenstone from Republic Mountain. 



The other components of these rocks are identical in form and nature 

 with those of the jjredominant schists. 



The greater schistosity of the Republic greenstones, as compared with 

 those near the Michigamme and 8purr mines, and their more crystalline 

 character, are accounted iov in the same manner as are the greater schistosity 

 and higher degree of crystallization of the western greenstones in general as 

 compared with the eastern greenstones, viz, by the fact that in the Republic 

 trough the mashing- of the igneous rocks, along with the sedimentary beds, 

 was greater than anywhere else in the Marquette district with the exception 

 of the Western trough. 



The general features of various rocks of the western greenstone knobs 

 and dike inasses have been described. Descriptions of the special features 

 of the different exposures, so varied are they, would consume more space 

 than would be justifiable in a discussion which is not purely petrographical. 

 A brief description of one or two exceptional phases, however, will be made. 



Some of the greenstone-schists deserve mention for the beautiful chlo- 

 ritoid found in them. This mineral has all the properties of true chloritoid. 

 It forms large plates that are pleochroic in greenish-blue and greenish-yellow 

 tints, and has an extinction of 1° to 3°, and often more. Lane, Keller, and 

 Hobbs (see Chapter I, pp. 129, 148) have described this chloritoid very fully. 

 But Lane and Keller state that "all the Michigan chloritoids, so far as yet 

 known, occur in altered arkoses or similar rocks." In the present instance, 

 and in some others to be mentioned later, the rocks in which the chloritoid 

 exists are (piite certainly mashed eruptives. They consist of brownish-green 

 biotite, large flakes of the chloritoid mentioned, crystals of clear and almost 

 colorless epidote, and sn^all grains of magnetite, forming areas between 



