150 THE GEOLOGY OP MINNESOTA. 



[Porphyry te. Amygdaloid. 



No. 58. PORPHYKYTE. (Diabase.) 



Duluth. On the lake shore, a little west of the line between ranges 13 and 14. Extends 15 rods. Some- 

 times rises in a bluff twenty-five feet high. 

 Kef. Annual Report, ix, page 20. 



Mcy. A dark-brown, fine-grained rock consisting of small feldspars in a darker 

 background. Porphyritic plagioclases, of a light-brown to pinkish color, are abundant. 

 Minute bright red spots or stains are seen throughout the rock. 



Mic. This rock is quite similar to some already described (Nos. 7BC, 46). The 

 \Krgeplagioclase phenocrysts are embedded in a groundmass which consists of minute 

 lath-shaped feldspars in a mass of alteration products- nifii/iiHifc, chlorite, rti/ritr, 

 quartz and red material (ln'ind/itf). It is not improbable, although not at all certain, 

 that augite was originally present in the groundmass. There are some areas now 

 filled by chlorite and magnetite which probably represent olicinc. A red hematite 

 stain pervades the rock in places and extends into cracks in the phenocrysts. 



One section. 



Age. Cabotian. u. s. o. 



No. 59. AMYGDALOID. 



East Duluth; east of Tiseher's creek; small outcrop in the shingle of the beach, having an apparent dip west. 

 Ref. Annual Report, ix, page 20. 



Meg. The amygdules are filled with green epidote, white calcite and quartz, 

 and the rock itself is porphyritic with red feldspars. It is also veined by quartz and 

 epidote, making an attractive rock; contains fragments of a foreign rock, apparently 

 a tuff. 



Mic. The large reddened feldspars are sometimes zoned, and twinned polysyn- 

 thetically. The microlites are 'adjusted to greenish areas, which have apparently 

 resulted from a change of pyroxene to chlorite. The minerals are all stained with 

 hematite. 



Age. Cabotian. N. H. w. 



No. 60. PORPHYKYTE. (Dinhum'. ) 



East Duluth. From a little rocky point and round the bay immediately west of London. This rock 

 forms a high and continuously rocky shore. 

 Ref. Annual Report, ix, page 20. 



Meg. Reddish-brown, finely crystalline, frequently jointed, hardly amygda- 

 loidal or porphyritic, becoming amygdaloidal in patches and coarsely concretionary, 

 containing nests of dog-tooth spar. 



Mic. Lath-shaped feli/.yt/rx are abundantly distributed in an ophitic sur- 

 rounding, which, while possibly originally pyroxene, is now a reddened secondary 

 mineral which polarizes in the aggregate manner characteristic of a mineral wholly 

 altered. The chloritic products usual after such a decay of pryroxene are almost 



