422 THE GEOLOGY OP MINNESOTA. 



[Basalt. Diabase. 



Meg. Resembles an altered volcanic debris. It is light-colored and crumbling. 



Mic. There is a profusion of microscopic grains, some of which (zoisite?) are 

 of very low birefringence, and others, though fewer, which are bright, and extinguish 

 parallel to their length (mica? and thomsonite?), while large areas that were once 

 occupied by some crystals (augite?) are now green with rings and rosettes of chlorite 

 (delessite?). 



Age. Puckwunge (basal Potsdam?). N. H. w. 



No. 562. BASALT (ivith olivine). 



From a niche in the coast-line south of the east end of Siskiwit lake. This is under the sandstone that 

 forms the peninsula south of Siskiwit bay, but above the conglomerate of the Island mine. Rises from the 

 water with dip to the south. Associated with harder and also epidotic rock in patches. 



Ref. Annual Report, x, pages 52, 53. 



Me (/. The specimen shows a homogeneous, somewhat decayed, medium-grained 

 brownish rock, similar to numerous Keweenawan diabases. 



Mic. An ophitic structure is apparent. There is not only fresh angite, but a 

 ferruginous much altered mineral whose relation to the feldspar is ophitic, suggesting 

 its derivation from augite or from glass; it resembles otherwise the brownish 

 hoirlingite which is derived from olivine. Careful inspection, however, shows that it 

 has resulted from the alteration of both. When it occupies the areas of original 

 olivines it has some translucency, and even birefringence, and its color is like that 

 of hematite or it is replaced by an isotropic or faintly polarizing substance which 

 between the nicols is sometimes blue (pennine?). When it is not derived from 

 olivine it is formed of an irregularly disseminated magnetite powder, which is 

 aggregated frequently in such quantities that the entire mass is opaque. When not 

 abundantly massed it is seen to be in an isotropic or glassy matrix which is 

 undoubtedly a residue from the magma. The feldspar is considerably penetrated 

 with epidote. 



Two sections. 



Age. Manitou. N. H. w. 



No. 563. DIABASE. (Altered.) 



From one of the trap layers alternating with sandstone and shale at the entrance of Chippewa harbor, 

 Isle Royale. 



Ref. Annual Report, x, pages 52, 53. 



Meg. Much changed, brown, apparently amygdaloidal and pseudamygdaloidal, 

 spotted white with calcite. Shows also a finely radiated green mineral, apparently 

 some form of chlorite, like delessite. The rock is firm. 



Mic. Feldspar is filled with gray and yellow epidote or with general saussuritiza- 

 tion. Augite is entirely lost by the severe alteration to which the whole rock has 

 been subjected. No olivine is distinguishable. In place of these primary minerals 



