142 THE GEOLOGY OP MINNESOTA. 



[Basalt. 



Mic. The description of No. 70 applies well to this rock, except that it is less 

 decayed. It might be added that the generally fresh condition of the rock, and the 

 contrast between the undecayed augite (sometimes diallage) and that which is 

 presumed to be a changed condition of the pyroxenic element renders it doubtful 

 whether the pyroxenic element has really suffered this change. It provokes the 

 query why such change should attack some of the grains and continue to total 

 obliteration, and leave the rest wholly fresh. The augite in this section shows 

 numerous grains cut so as to show optic axes and bisectrices in the field of the 

 microscope, and occasionally an optic normal. The decayed, now chloritic, substance 

 may be a condition of the residuum of the magma. 



Olivine is quite common in the slide and is distinguishable from the augite by 

 the fact that its boundaries encroach on all the other minerals, by its slightly 

 darker tint of yellow in common light and by the irregularity of its cleavages. 

 It is further distinguished, when a bisectrix can be found perpendicular to the plane 

 of the slide, by the axial angle being about 90. In the case of augite the smaller 

 axial angle is much less than 90, and the larger one much more. One such bisectrix 

 (//,,) occurs in the slide examined. The olivines are quite fresh in this rock, but in 

 No. 70 they are much altered. 



The magnetite is angular and fresh, as if an original secretion from the magma. 



One section. 



Aye. Manitou? (but possibly a feeder to the Cabotian sill No. 44'.) 

 Remark. The freshness of some of the augite in the thin section, contrasted with 

 the completeness of the alteration in some of the other ophitic, greenish substance, 

 suggests the possibility that the alteration product does not arise from augite. Its 

 evident late date in the generation of the various minerals shows that it can have 

 ai-isen, if not from augite, only from a non-differentiated portion of the original 

 magma. Again, the freshness of the olivine rather precludes the supposition that 

 the augite may have suffered so much. N. H. w. 



No. 48. BASALT. 



Duluth. Second street, corner Fourth avenue east. This runs from in front of C. Markell's house, under 

 the Hayes block. It is a large member, at least 150 feet thick, and apparently falls between Nos. 7 and 6C. 

 Ref. Annual Report, ix, page 18. Annual Report, x, page 38. 



Meg. Close grained and firm, hard, bluish gray to black, heavy, not visibly 

 amygdaloidal, but finely and sparsely porphyritic. 



Mic. The groundmass is made up of microlitic feldspars and magnetite. 

 Evidently several other minerals are also present, but they are too fine, and the 

 section (too thick at best) is too much decayed in general to warrant any attempt to 

 differentiate them. The section shows a portion of a tabular crystal of a porphyritic 



