PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 813 



Greenwaeke.] 



litic structure was taken by the quartz, and that it also crystallized as hematite before 

 that structure was acquired. Thus, within a triangular space filled otherwise by 

 interlocking grains of quartz, surrounded by what appears to be amorphous iron ore, 

 there are several crystals of hematite standing on the surrounding hematite border. 

 These crystals are sufficiently long to reach about half way across the triangular space, 

 and they cut through the quartz grains idiomerphically, each crystal thus penetrating 

 from two to four of the quartz grains. In the slide are a number of illustrations of 

 this priority of the hematite. One section. 



Age. Archean (Lower Keewatin). 



liemark. This priority of the iron ore, in its present form, over that of 

 the quartz, in its present form, may be united with notes on the condition of the 

 jaspilyte where it has been modified by the gabbro contact at Disappointment lake 

 (Nos. 2199-2201). N. H. w. 



No. 1962. GREENWACKE. 



North slope of the North ridge, Soudan. 

 Ref. Annual Report, xxii, page 10. 



Meg. The rock has an approach toward argillyte. 



Mic. The slide, however, shows a rock much too coarse for argillyte. It is 

 essentially made up of basic debris, perhaps tuffaceous, in which great decay has 

 taken place. The old feldspars (and augites, if any existed) are hardly discernible, 

 but a chioritic and amphibolic and epidotic confused secondary product has taken their 

 places, showing a sprinkling also of hematite. One section. 



Aye. Archean (Lower Keewatin). N. H. w. 



No. 1966. GREENWACKE (ivith cordierite? ) 



Crossing of Lake street and Piedmont avenue, Duluth. 



Ref. Annual Report, xxii, page 16. (Compare Nos. 44 and 1708.) 



Meg. Typical homogeneous specimen of the " black rock," so-called. Appearing 

 like a basalt. 



Mic. The rock is very fine grained, but in high powers it is seen to be composed 

 of the same elements as a greenwacke, i. e., amphibole, old feldspars nearly destroyed, 

 magnetite (or hematite) quartz in angular and subangular grains, all more or less 

 enshrouded in an isotropic mantle of chlorite. The rock might pass for an Archean 

 greenwacke, or even for a volcanic tuff. One section. 



Age. Animikie. 



Remark. The supposed " old feldspars " are possibly not feldspar, but cordierite. 

 They frequently have parallel extinction and are crowded with inclusions, but being 

 very small and indefinite, it is not safe to rely on them as cordierite. N. H. w. 



