PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 309 



Tuff. Gabbro.] 



spars are set in a mosaic of finer grains, probably caused by peripheral granulation of 

 the larger grains; the section, however, is too thick to show this point with certainty. 



One section. 



Age. Archean. u. s. o. 



No. 307. TUFF(?) 



At a point about a mile west of the last, on the south side of the Gunflint river, west of the "narrows." 

 Bef. Annual Report, ix, pages 81, 83. 



Metj. Apparently an irony and carbonaceous shale, pyritiferous, firm and heavy, 

 with flinty nodules, exposed perhaps two feet, nearly black. 



Mir. The rock is fragmental and confused, of varying texture and grain, a 

 considerable percentage being opaque black. The aspect is that of a finely vesicular or 

 scoriaceous, mainly fragmental, rock whose cavities have become filled with calcite, 

 or with hematite and calcite set in a dark, even opaque, ground work. Some rounded 

 areas are much finer than others, and some of the fragments are very fine and 

 scarcely polarize light. Sometimes quartz shares in the fillings of the minute cavities. 



Three sections. 



Age. Taconic (near the base of the Animikie). 



Remark. The nature of this rock cannot be determined from the data at hand, 

 except that it may be affirmed that its origin was through the accumulation of a 

 peculiar fragmental debris. It is more nearly like a Carboniferous tuff of King's 

 county, Ireland (No. I, 1397 of the series of the Survey of Great Britain), of which 

 thin sections have been made from samples furnished by Sir Arch. Geikie, than any 

 rock with which it is now possible to compare it. It is darker than that, and its 

 ground work is less greenish. There are, however, isotropic, apparently glassy, por- 

 tions which have a finely fluidal structure. 



It is comparable, stratigraphically, with the glassy breccia described by Williams 

 at Sudbury, Canada,* and with the tuffs of the Penokee range described by Van 

 Hise.f N. H. w. 



No. 308. GABBRO. 



"The trap of the country; south side of Gunflint lake; sec. 24, T. 65-3 W. 



"Compare Nos. 721-727." 



Ref. Annual Report, ix, page 81; Annual Report, x, page 86. 



Meg. A fine-grained, dark-gray, diabasic rock. 



Mic. The section shows a gabbro of rather fine grain, and considerably altered. 

 The feldspar is much kaolinized, but a number of comparatively fresh grains remain. 

 Some of these show equal extinction angles on each side of the albite twinning line 

 running up as high as 30, thus indicating l<il>m<l<-if<'. The augite is in part fresh, 

 and part altered, the alteration products being chlorite, hornblende and biotite. 



'Bulletin of the Geological Society of America, vol. iii, p. 138. 

 ^Bulletin of the Geological Society of America, vol. iv, pp. 435,438, 1893. 



