874 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



Meg. Same rock as No. 2254, but showing roundish, light-colored mineral 

 aggregations that give it the appearance of being amygdaloidal. 



Mic. The ophitic structure is preserved although the augite is wholly altered 

 to uralite. Considerable masses of coarsely cleaved calcite are conspicuous in this 

 rock. The feldspar is so altered that its space is occupied by granular secondary 

 minerals, and these are so abundant that it is wholly impossible to determine even 

 the direction of extinction of the original feldspar. Amongst these secondary 

 minerals, probably epidote, chlorite, calcite and leucoxene are the most abundant. 

 Spicules of hornblende also sometimes pass through these areas. In the slide there 

 is no evidence of amygdaloidal structure. One section. 



Age. Igneous, cutting Upper Keewatin conglomerate. N. n. w. 



No. 2256. DIABASE. ( Uralitizc'l. ) 



Same rock, same place. 



Ref. Annual Report, xxiv, page 79. 



Meg. Same rock, showing the white fillings. 



Mic. The section cuts one of the white fillings. The structure of the mineral 

 composing this substance is nearly parallel, fibrous or lamellar, but divergent 

 and blending and confused, having a predominant positive elongation, but with 

 some fibres of a negative elongation. It has a very low double refraction, and an 

 extinction angle of about 10 on the elongation of the fibres. About the periphery 

 of this mass is much granular epidote. From the single section at hand it is not 

 possible to name this species with certainty by the foregoing optical characters 

 alone; but, except for the small angle of extinction, it agrees well with apoplnjllite. If 

 the interferences and anomalies which are known to appear in that mineral be 

 allowed for in this section perhaps there is no obstacle to that identification. While 

 some of the fibres have a small angle of extinction others have parallel extinction, 

 and in others it is shifting and imperfect, which can be attributed to overlapping 

 twins, or combinations which are somewhat oblique or distorted. Compare No. 2258. 

 One section. 







Age. Greenstone (igneous) cutting the Upper Keewatin. 



Remark. Later this zeolitic mineral was repeatedly subjected to the Boricky test 

 with hydrofluosilicic acid. No distinct crystalline forms were produced, but the field 

 was covered with minute irregular rods and globules, the former branching and crooked. 

 Through these forms feeble double refraction could be perceived by reason of extinc- 

 tion recurring at certain points on rotation, but for the most part the crystals were 

 confused and imperfect. These forms were evidently fluosilicate of lime. There 

 was no appearance of cubic forms, and hence the mineral may be free from potas- 

 sium. >. H. w. 



