THE IGNEOUS ItOCKS. 501 



appear to have played the more important rok^ In these rocks the am[)hi- 

 bole is dark-green and compact. It is never pale-green and fibrous. More- 

 over, chlorite is rare, except locally, while fresh brown biotite and grains of 

 quartz are abiTudaiit, and in many cases there has been produced a mosaic 

 of the latter mineral and albite, as in the case of the hornljlendic schists of 

 the Basement Complex. 



These differences between the eastern and the western greenstones 

 mav be explained by the differences in form and in the geological conditions 

 under which the rocks are found. The eastern knobs are irregular in 

 shape and are boss-like in their features, while the western knobs are linear 

 in shape and dike-like in their general features. The former were able to 

 withstand stress more successfully than the latter, and so have suffered less 

 d-s'uamic metamorphism than these. Moreover, in the western jjart of the 

 Marquette district the folding and mashing of the formations Avere more severe 

 than they were in the Ishpeming-Negaunee area. (See Metamorphism, 

 Chapter VII, pp. 573-575.) Consequently, as a rule, the western green- 

 stones are more schistose than the eastern ones. Tliey are dai-ker in color, 

 fresher in appearance, and more crystalline-looking. Often large, brilliant, 

 dark-green or black ampliiboles lie in a dark-green groundmass, through 

 which small, sparkling crystals of the same mineral are thickly strewn, with 

 their longer axes in the planes of schistosity. In their macroscopic features 

 these rocks resemble very closely schistose diorites and camptonites. 



Under the microscope all their sections are fresh-looking. The weath- 

 ering products so noticeable in the eastern gi'eenstones are rarely observed. 

 Biotite is their characteristic component. It occurs as large and small 

 plates of a deeji reddish-brown color, like that in the micaceous schists of 

 the Basement Complex. It is derived very largeh^ from the plagioclase. 

 In those specimens in which the plagioclastic nature of the altered feldspar 

 is still clearly apparent, small brown biotite flakes, little spicules of horn- 

 blende, and granules of epidote are scattered in large quantities through 

 the feldspathic substance, and here and there quartz also is present. Upon 

 further alteration of the plagioclase the quartz becomes more and more 

 abundant, and sometimes secondary albite is formed. In the final stage of 

 the change all the plagioclase has been replaced by an aggregate of biotite, 



