108 THE GEOLOGY OP MINNESOTA. 



[Porphyry to. Tuff. 



No. 8. PORPHYRYTE. (Amygdaloid*//.) 



Duluth. Separated near the wall of contact of No. 6. 



Kef. Annual Report, ix, pages 12, 13. Annual Report, x, page 140. American Association for the 

 Advancement of Science, vol. xxx, page 164. 



Meg. Dense, dark colored, with indistinct small porphyritic feldspars, and in 

 some places amygdaloidal cavities filled apparently with chlorite, or chlorite 

 and quartz. 



Mic. The prevailing mineral is a lath-shaped feltlx/xir of a brownish red color, 

 apparently one of the first of the minerals to solidify. 



1'ijro.miic element is not discoverable, as such, but its former presence is 

 indicated by chlorite, and by magnetite. The latter is in numerous cubic and 

 irregular forms, which are seen within the fine quartzes distributed in the body of 

 the section. 



The amygdaloidal spaces are occupied sometimes by chloritic substance, and 

 sometimes by quartz, and often by both in more or less zonal arrangement. 



J't/ritv is occasionally seen in the borders of the amygdules. 



Age. Probably Cabotian. 



One section examined. N. H. w. 



Remarks. Another form, also numbered 8, is much less amygdaloidal, and more 

 porphyritic. It is very fine grained, compact and varies in color from dull black to 

 dark brown. None of the constituents of the mass of the rock can be distinguished. 

 Where the color is black the rock is homogeneous, but as the color becomes brownish 

 numerous rather indistinct small dark blotches are seen. A few irregular areas of 

 reddish feldspar occur; one of these is three-eighths of an inch across. There are also 

 a few larger areas where the rock is colored reddish; in the centre of these areas is 

 often a yellowish substance, probably epidote. One of these areas, or large blotches, 

 is seen to have a centre of yellow material, outside of which is a distinct narrow black 

 band; and beyond this is a red band, which passes into the general color of the rock. 

 The black portion of the hand specimen closely resembles, macroscopically, some of 

 the black flinty slates of the Animikie. 



Still other forms of this rock are sparsely porphyritic with a feldspar which has 

 a tendency to become red. Compare Part III. r. s. G. 



No. 8A. TUFF. (Inclusion in No. <v. ) 



Kef. Annual Report, ix, page 12. Annual Report, x, page 140. American Association for the Advam-,-- 

 ment of Science, vol. xxx, page 163. American Geologist, vol. xviii, pages 211-213. 



This is similar in all respects to the fine rock seen in No. 7B, as foreign 

 inclusions. It is nearly totally dark between nicols, but shows numerous fine angular 

 quartz grains. With one nicol and the condenser lowered, a varied structure comes 

 out to view. It is difficult to describe it. The glassy grains, charged with inclusions, 



