PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 103 



Amygdaloid. Tuff.] 



that this may be one of the varied results of the action of the basic eruptives on the 

 varied nature of the elastics with which they came into contact; although it is more 

 likely to have originated from a quickly cooled basic magma which at first took the 

 character largely of zirkelyte, a name applied by Wadsworth to a microlite-charged 

 basic glass in such conditions, afterwards partially or wholly devitrified. (Bulletin 

 ii, Minnesota Survey, page 30.) N. H. w. 



No. 7A. AMYGDALOID. 



Duluth. Apparently overlies No . 7. 

 Sef. Annual Report, ix, pages 12, 17. 



Meg. Nearly black, amygdaloidal, at least with cavities nearly or wholly filled 

 with segregated minerals. The most conspicuous of these secondary minerals is 

 yellowish green, resembling epidote; another is dark green, and appears in the form of 

 radiated coatings, and resembles delessite. Pyrite also is sparse. The structure is 

 very fine, but somewhat irregular and globular. 



Mic. This rock is like the last in its essential characters. In some places the 

 section appears reddish, by reason of the abundance of the red feldspars. In others 

 it is porphyritic with fine felclspathic microlites. But the whole rock is much decayed, 

 and is seamed by irregular transparent veins and threads, which consist largely 

 of quartz. 



One section examined. 



Age. Probably of the Cabotian eruptives of the Taconic. N. H. w. 



No. 7B. TUFF(?) 



Duluth. This rock is wrought, or was in 1878, in the alley between First and Superior streets, and Fourth 

 and Fifth avenues cast . 



Ref. Annual Report, ix, page 12. 



Meg. This rock is brownish, but spotted with greenish amygdules and with 

 inclusions of some foreign rock. This foreign rock is of a dun color, rather compact but 

 appears itself like a fragmental rock. On weathered surfaces it is pitted from the 

 oxidation of pyrite. These foreign pieces vary from the size of a pea, or perhaps a 

 pinhead, to about two inches in diameter, which is about the size of a mass attached 

 to the museum sample preserved. They are scattered heterogeneously in the 

 amygdaloidal portion of the rock. 



Mic. A section of the matrix which embraces the foreign pieces shows a much 

 decayed condition. There are reddish feldspars which come from the first epoch of 

 consolidation, some of them plainly twinned polysynthetically. They are charged 

 with alteration products, such as epidote, pennine, pyrite, hematite ( ?) magnetite, and 

 apparently zircon, although these minerals are not always arranged so as to prove 

 they are included in areas formerly occupied by feldspars. They, indeed, fill up the 



