200 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



No. 133. DIABASE (with <iiti-i-<-<i mut/mi/fic I/IUNS). 



From the second island in Beaver liny, counting from the west. 



Ref. Annual Report, is, page 34; Bulletin ii, pages 59, 70, plate II, figure 1. 



Meg. A dark-gray, heavy basic eruptive, apparently from the great sheet of 

 trap which all along from Silver creek and Encampment island carries masses'of feld- 

 spar. It glitters with metalloidal reflections of a pyroxene which embraces other 

 minerals ophitically. Its coarse granular structure and its freshness here cause it to 

 resemble the gabbro of which pieces are embraced in it at other points in the vicinity. 

 It resembles No. 132. Compare Nos. 221, 222 and 223. 



The feldspar is very fresh and is taken for labnulorite. A small crystal of the 

 feldspar which is entire, represented below, lies surrounded by the green, massive 

 chlorite (?) and is cut exactly parallel to 010. It is not twinned, so far as can be 

 seen. It has extinction at 37. By comparing this with the table of 

 extinctions given by Rosenbusch (translation by Iddings, page 300) 

 it is evident that the feldspar is more basic than usual for labradorite, 

 and that the species is near anoiiliitc. Fouque, however,gives ninn-lhHc 

 . at 41. There are in the section several Carlsbad twins, also; in one 

 case cut nearly perpendicular to the zone of symmetry, as shown by 

 the four points of equal illumination for all the bands, each of the 



FIG. 10. 



twins being also twinned on the albite plan. 



The pyroxene element is strongly refracting and doubly refracting, appearing 

 like augite with a tendency toward diallage owing to the secondary cleavage which 

 is seen in nearly all the grains, parallel to which extinction takes place; indeed, there 

 seems to be but one crystal of this pyroxene in the slide, since extinction in this 

 mineral is simultaneous throughout. There is a remarkable contrast, as noticed -in 

 several other instances, between this pure pyroxene and the greenish substance 

 which is generally supposed to be the product of its alteration, and in some instances 

 this contrast, in this rock, is between two adjacent masses which have no gradation 

 toward each other. The green substance in this case does not result from a change 

 in the pyroxene, but is more than likely to be a portion of the residual magma. This 

 green substance is in the feldspar surrounded in a manner like that of the pyroxene, 

 and it is sometimes irregularly disseminated sparsely through the feldspar, from 

 which it is separable as clearly as from the augite. 



Owing to the fact that by Wadsworth this pyroxene element is called enstatite 

 (Bulletin ii, page 70), a more particular examination was made. Outwardly, and 

 microscopically, as Wadsworth remarks, its appearance in common light is identical 

 with that of diallage, which is a secondarily (?) cleaved form of augite; and but for 

 the fact that he has pronounced it enstatite there would be no suspicion that it could 



