BASEMENT COMPLEX OF STUEGEON RIVER TONGUE. 465 



THE AMPHIBOLE-SCHISTS. 



In addition to gneissoid g-ranites, the southern area of the Basement 

 Complex contains a number of ledges of dark-colored schistose rocks. 

 These, in some instances, are cut by dikes of granite similar to the granite 

 already described. 



These dark schists may be classed as greenstone-schists and as horn- 

 blende-schists. The former are heavy rocks, with dull greenish-gray luster 

 and distinct scliistose structure. They resemble closely in their micro- 

 scopic as well as in their macroscopic features the schistose dike greenstones 

 to be referred to later. They are doubtless altered or squeezed diabases or 

 gabbros. 



The hornblende-schists are usually tine-grained, bluish-black rocks, 

 with a very even schistosity, closely resembling slaty cleavage. On the 

 surfaces of cross fractures may be seen long slender prisms of glistening 

 black hornblende arranged in as distinct lines as the lines of particles in an 

 evenly bedded sedimentary rock. Often the cleavage surfaces are coated 

 with thin layers of golden-yellow mica scales. In most specimens there 

 may also be noticed a fine banding parallel to the foliation. 



In thin section these hornblende-schists differ from the schistose dike 

 diabases and from the greenstone-schists, referred to above, in the presence 

 of large quantities of quartz, and of some biotite, and to some extent in 

 structure. The greenstones owe their schistosity to the flattening of their 

 components, while in the hornblende-schists this structure appears to be due 

 largely to the crystallization of the hornblende in elongated prisms with 

 their major axes parallel. The parallel ai'rangement of the amphibole in 

 the latter rocks is thus much more pronounced than in the schistose 

 greenstones. 



The hornblende-schists are composed of hornblende, quartz, biotite, 

 plagioclase, magnetite, and sphene. 



The hornblende is in long prisms of the usual yellowish green color. 

 The mineral is compact, but it is full of inclusions of quartz grains similar to 

 those constituting a large part of the matrix lying between the amphiboles. 

 It was evidently formed in situ as an original crystallization, and not, like 

 much of the hornblende of the schistose greenstones, by the alteration of 

 augite or of some other component of a basic crystalline rock. The biotite 



MON XXXVI 30 



