128 THE GEOLOGY OP MINNESOTA. 



[Zirkelyte. 



fissures, and hence appears more frequently in lines. The border is also always 

 heavily charged with magnetite as well as these fissures, leaving one or more 

 central areas free from magnetite; while in the decay of augite the secondary mag- 

 netite appears more uniformly throughout the grain often as a fine powder or in 

 particles of some size. (3) The greenish substance that takes the place of these 

 minerals is apparently, in chemical composition, the same in one as in the other, 

 but in the case of olivine the alteration product is apt to show a finely fibrous 

 structure. (4) In this slide a further noticeable difference consists in the presence of 

 considerable hematite with the magnetite, in the changed augite, and the almost 

 entire absence of this mineral in those changed grains that are unmistakably derived 

 from olivine. These differences are not always pronounced, and do not always 

 coexist, but by one or more of them usually the original olivinitic nature of the rock, 

 pro or con, can be determined. 



Hematite, as already noted, is quite abundant, not only in the form of fine powder 

 staining the feldspars, but in company with the magnetite in the grains that have 

 resulted from the change of the pyroxenic mineral, where it appears as brownish 

 red microscopic crystalline scales and groupings. Such hematite groupings appear 

 dull red in ordinary light, both transmitted and reflected, but when the stage is 

 rotated in strong reflected light they show minute reflecting surfaces which reappear 

 at the proper angles. 



Calcite is seen in isolated patches. 



Two sections examined. 



Age. Probably Manitou. N. n.\v. 



No. 82. ZlRKLEYTE. 



Dulutb. At the lake shore, east of Minnesota point, on the east side of a fault. 

 Ref. Annual Report, ix, page 15. 



Meg. A very fine-grained rock, dark brown in color, spotted with clusters of 

 green epidote or epidote and chlorite. 



M ic. This is a very fine basalt in which the minute plagioclasex are lath shaped 

 and rarely twinned beyond two lamellae, and embraced ophitically in the now 

 changed matrix, which was probably glassy. 



Magnetite is abundant. 



With the epidote and chlorite filling the amygdaloidal (?) spots, is a little i/iuiz. 



One section examined. 







Age. Cabotian. N. H. w. 



No. 32A. ZIRKELYTE. ( Weatltered modification .) 



Duluth. Taken from No. 32 near the top of the bluff, where it becomes reddish-brown and breaks into 

 angular blocks of a few inches. 



Ref. Annual Report, ix, page 15. 



