PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 121 



Calcite andepidote. Zirkelyte.l 



fine linear crystallites are common. Magnetite in fine grains is scattered everywhere. 

 A dirty greenish or grayish mineral can be seen abundant in the vicinity of the 

 magnetite, resembling that seen in No. ISA, probably the result of alteration of the 

 magnetite, and indicating its titaniferous nature. 



The rock contains also some quartz, in minute granules embraced in the general 

 trachytic areas, also a mono-axial mineral darkening with the nicols which appears 

 to be aptitife. 



One section examined. 



Age. Cabotian. 



Remarks. This rock has been grouped by the Minnesota Survey in the series of 

 " red rocks." The microscopic characters seem to indicate that this is not correct. 

 The following chemical analysis* shows the rock is rather lower in silica than the 



most of those rocks. 



l 



SiO 2 57.50 



TiO 2 



A1 2 O 3 13.29 



Fe 2 O 3 11.62 



FeO 4.54 



MnO 



CaO 6.12. 



MgO 1.63 



K 2 O 0.80 



Na 2 O 1.85 



H,O - 1.48 



Total, 98.83 



N. H. W. 



No. 19A. CALCITE AND EPIDOTE. 



Duluth. From a large concretion in No. 19. 

 Ref. Annual Report, ix, page 14. 



Meg. A mass of epidote, calcite, a light-gray, soft rock and a dark greenish rock. 



Mic. The section is largely a mass of calcite grains in which is considerable 

 epidote in small crystals and grains and some quartz. A little chlorite is present, as 

 is also magnetite. Scattered through the calcite is a gray opaque substance, which 

 at times is arranged so as to give the calcite a zonal structure. 



One section. 



Age. Concretion in Cabotian rocks. u. s. G. 



No. 20. ZIRKELYTE. ( Aimjgdaloidnl. ) 



Duluth. A tougher condition of No. 19. Embraces angular masses of No. 21, and extends about forty feet. 

 Ref. Annual Report, ix, page 14. 



Meg. A dark aphanitic rock containing a few red porphyritic feldspars. The 

 rock is thickly strewn with small vesicles which are in part empty and in part 

 contain epidote, and sometimes quartz or calcite.. 



*Thirteenth Annual Report, p. 100. 



