134 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



Mic. The texture does not present the strong idiomorphic characters seen in 

 the feldspars of No. 38, yet it is an ophitic rock. The pyroxene is changed to the 

 usual decomposition products. Chlorite, magnetite and hematite abound. 



One section examined. 



Age. Cabotian. N. H. w. 



No. 41. DIABASE (with olivine, in basaltic columns}. 



Duluth. Across a little bay, east from the last. Basaltic rock, the columns dipping about 10 from the 

 perpendicular toward the northeast. Varies from a texture like that of No. 1C, to a finer grain, much like No. 

 43. Adjoins No. 50 on the east. 



Ref. Annual Report, ix, pages 17, 18. 



Meg. A tolerably fresh, medium grained, dark, basic rock, showing a polysyn- 

 thetic feldspar, magnetite and a pyroxene. 



Mic. The feldspar is labradorite, its extinction angles on opposite sides of a 

 twinning line being as high as 25 and 27|, and 31 and 33. 



Pyroxene is generally well preserved and exhibits a characteristic relation to 

 the feldspars. It is in the form of augite. Some of it was at first taken for olivine, 

 owing to its generation earlier than the feldspars, as evidenced by its independent 

 outlines. In some instances the earlier and the later augite grains are associated in 

 groups, in which their different relations to the surrounding feldspars are contrasted. 

 In order to determine these earlier grains more certainly, the slide was uncovered, 

 washed with turpentine to remove the Canada balsam, then with alcohol, and lastly 

 with water. It was then immersed in hydrochloric acid during about eighteen 

 hours, in order to convert it, if olivine, to gelatinous silica. The slide, after washing 

 again in water was then covered with iodine green and left for twenty minutes. 

 When examined for gelatinous silica, none was found; at least those augites which 

 most clearly maintained their independent outlines were found unattacked, and as 

 clearly polarized light as before the test. Other grains absorbed the color, some of 

 them being probably olivine and others remnants of the magma uncrystallized. The 

 latter were originally greenish, and chloritized. This is the first rock in which we 

 have distinguished augite amongst the early generations from the magma. It is still 

 more interesting that it was also later than some of the feldspars. In one instance, 

 indeed, a single augite grain can be noticed, which is ophitic toward one feldspar 

 crystal and idiomorphic toward another. 



Olivine. There appears to be some later olivines which can hardly be distin- 

 guished from pyroxene, except by a careful noting of the cleavage and extinctions. 

 They are both colorless or faintly straw yellow, and their double refraction, in a 

 section of ordinary but unknown thickness (though less than 0.03 millimeters), is so 

 nearly the same that the interference colors are not characteristic. Two sections 

 were found, however, perpendicular to an optic axis and showing two cleavages per- 



