PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 703 



Peridotyte. Gabbro. Greenwacke.] 



warrants the hypothesis that the pyroxenic element depends on the proximity of the 

 gabbro. It would then be left to inquire, on the hypothesis that the Pewabic was a 

 pure quartzyte originally, whether that difference is due to infiltration of hypersthene, 

 etc., into the quartzyte, or the infiltration of quartz into the base of the gabbro, 

 accompanied by an alteration of its ferromagnesian minerals, and especially of 

 augite, into hypersthene, and the entire loss of the feldspars. But as the Pewabic 

 quartzyte is only a changed jaspilyte varying to muscovadyte, it must have contained 

 originally more or less greenstone debris capable of giving origin to all these ferro- 

 magnesian minerals in situ. N. H. w. 



No. 1365. PERIDOTYTE. 



A little northeast from the " narrows " of Gabemichigama lake, on the south shore. 

 Ref. Annual Report, xvi, pages 93, 123. 



Mi-;/. Heavy, banded, a part of the " olivinitic iron ore." 



Mic. This rock consists almost wholly of olivine (fayalite), with a little 

 hrjpersthene and maynetite. There is also a little grunerite, distinguished by its 

 multiple twinning, its higher refraction (tested by the Becke method) and its higher 

 double refraction. The surface roughness of the grunerite is visibly less than that 

 of the olivine, which it pierces, and than the hypersthene, yet the fine Becke line 

 always moves toward the grunerite on raising the objective. 



Examining another slide, it appears that there is a little quartz in this rock. It 

 is in round grains in the hypersthene, or indents its borders. Two sections. 



Age. Cabotian (changed Keewatin). N. H. w. 



No. 1366. GABBRO. (Muscovadyte.) 



At the narrows of Gabemichigama lake. Embraced in the Pewabic quartzyto No. 1364. Compare Nos. 

 1341 and 1344. 



Ref. Annual Report, xvi, pages 93, 123. 



Meg. Fine-grained gabbro. 



Mic. Playioclase, augite, olivine, magnetite are the essentials in this slide, 

 without an ophitic structure in the main; some of the augites are small, rounded, 

 and apparently as early as the feldspar. The magnetite is in rounded masses, and 

 in larger branching parts, frequently embracing parts of the augite. The feldspar 

 is generally clouded with a dust of minute crystalliths which it is impossible to 

 determine, but which is in part apparently pyroxene and biotite. The latter also 

 appears in larger masses independent of the feldspar. One section. 



Age. Originally a layer in the jaspilyte of the Keewatin (now granular gabbro 

 in the Pewabic quartzyte, dating from the gabbro revolution). N. H. w. 



No. 1367. GREENWACKE. 



Various samples (10) obtained in the ascent of the hill (or mountain) from the southwest shore of Gabe- 

 michigama lake, near the narrows, intended to show the features that might be considered of sedimentary origin. 

 Ref. Annual Report, xvi, pages 93-95, 97-99, 123. 



