PETROGRAPHIO GEOLOGY AND DESCRIPTIONS. 809 



Diabase. Amphibolyte. Tuff.] 



These are all (except magnetite) secondary minerals, derivable from a decay 

 and rearrangement of the elements of a basic rock, or from basic volcanic tuff, as 

 often illustrated in Minnesota. One section. 



Aye. Keewatin. N. H. w. 



No. 1918. DIABASE. 



Lower Quinnesec falls ; excavated for the channel for running logs ; forms the barrier of the falls. 

 Bef. An mal Report, xxii, page 13. 



Mcy. Coarse, gabbro-like, but much altered, often spotted with white; perhaps 

 as a dike. 



Mic. The rock shows an ophitic relation between the hornblende (altered from 

 augite) and the original feldspars, the latter being too much decayed for specific 

 determination. There were some larger feldspars, but they are altered to zoisite 

 and calcite, and otherwise obscured so that they cannot be determined. A part of 

 the amphibolic mineral is fibrous actinolite. Apatite is in crystals of the first 

 consolidation. One section. 



Aye. Keewatin (dike in?) N. H. w. 



No. 1919. AMPHIBOLYTE. ( Garnetiferaus. ) 



At a point in the first hill just south of the Republic mine, Michigan. 

 Ref. Annual Report, xxii, page 13. 



Meg. Structure dips north about 75. So-called actinolite-magnetite schist, 

 from the " lower Huronian." 



Mic. Many of the grains of the amphibole are multiple-twinned, but not all of 

 them. Mingled with them is much garnet. The magnetite in the slide is so small in 

 amount as to be negligible in giving name to the rock, there being but two or three 

 insignificant grains. One section. 



Age. (?) 



Remark. Named actinolite-magnetite-schist and anto-phyllite-schist, the pre- 

 dominant mineral of this rock was carefully examined by Lane and Sharpless, and 

 pronounced griinerite, and it has latterly been called griinerite-schist. Yet, according 

 to the analysis of Lane and Sharpless, it is placed recently by Hintze under cumming- 

 tonite. (Handbuch der Mineralogie, II, page 1230.) N. H. w. 



No. 1939. TUFF. 



From the dump of the Palms mine, Gogebic iron range, Michigan. 

 Ref. Annual Report, xxii, page 15. 



Meg. A rather fragile, greenish-gray rock, with quartz. 



Mic. A chloritic, isotropic substance forms the cement and background of this 

 rock. It has a varied aspect as to light and dark because of apparent small fragments 

 of different composition, with varying amounts of iron oxide. Sometimes a rusty 



