110 THE GEOLOGY OP MINNESOTA. 



[Diabase. Porphyryte. 



Chlorite is abundant, and sometimes its color between the nicols is blue, 

 indicating pninhu-. It constitutes independent angular masses, as if it had resulted 

 from alteration of some allotriomorphic mineral. In small flakes it is distributed 

 generally throughout the rock. 



Magnetite occurs in the same manner as the chlorite and is equally abundant. 



There may be other minerals in this rock, besides calcite, but the section made 

 is not favorable for their determination. 



One section examined. 



Age. Cabotian. 



Hi-marl: When collected this rock was supposed to have resulted from 

 metamorphism of clastic material, through the action of eruptive rock in the vicinity. 

 But the feldspar proves to be twinned, like a plagioclase, and the structure suggests 

 that the feldspars may have once been embraced by crystals of augite or by zirkelyte. 

 While resulting in part apparently from cooling from fusion, and subsequent 

 alteration, this rock, as a whole, may still have been composed of fragmental volcanic 

 debris, or it may have resulted from fusion of the elastics at the point of contact of 

 the rocks of thegabbro series, since the ophitic structure is not unquestionable. 



N. H. w. 

 No. 8C. DIABASE. (Fine.) 



Duluth. From a dike in No. 8. 



Ref. Annual Report, ix, page 13. Annual Report, x, page 140. American Association for the Advance- 

 ment of Science, vol. xxx, page 164. 



Age. A very fine grained, compact, black to brownish, diabasic rock. 



Mfg. The rock is composed of minute lath-shaped feldspars in a more or less 

 confused groundmass, which is largely stained to a brownish color. Aside from the 

 feldspars the rock is made up quite largely of magnetite, which occurs in very 

 minute grains, in irregular and often ill defined areas, and in needle-like forms. The 

 rest of the groundmass is quite fine grained and seems to contain chlorite, calcite 

 and probably some little quartz and muscovite. The groundmass appears to be all 

 secondary, except possibly some of the magnetite, but what its original nature was 

 is uncertain. The rock is here called provisionally a diabase, but it may have been 

 a basalt, i. e., its groundmass may have been of the nature of zirkelite. 



One section examined. 



Age. Probably a Manitou eruptive. U- s. G. 



No. 9. POKPHYRYTE. ( Amyydaloulal. ) 



Duluth. Not far from the base of Minnesota point. 

 Ref. Annual Report, ix, page 13. 



Meg. The rock is dark brown in color, presenting a dark aphanitic ground- 

 mass in which are reddish brown porphyritic crystals of striated feldspar and 



