494 THE GEOLOGY OF MINNESOTA. 



[Gabbro. 



tine is arranged in fine fibrous parallel bands occupying the entire surface. The 

 diallage contains much magnetite dust and grains arranged mostly along the cleavage 

 lines, as a product of alteration. 



" Plate XII, figure 2, shows the structure of some of the less altered olivines." 



One section examined. 



Age. Cabotian. u. s. G. 



No. 692. GABBRO (with hypersthene) . 



Forms a series of low hills and bluffs along the south side of Little lake. The rock resembles that of No. 

 691; extends also round the shores of Poplar lake. 



Ref. Annual Report, x, page 79; Bulletin ii, pages 59, 91, 92, plate II, figure 2. 



Meg. Medium grained, with gabbro aspect. 



Mic. The rock consists almost wholly of feldspar, hypersthene and augite, the 

 last usually in the form of diallage. 



The rock is fresh, and the relation of the hypersthene to the other minerals is 

 interesting, as it forms large crystals which extend more than across the whole field 

 of the microscope when a moderately low power is used (objective No. 3, of Nachet), 

 embracing in a poikilitic manner several crystals of each of the other minerals. It 

 was, therefore, the last mineral to be formed. The hypersthene is dichroic, having a 

 faint green color when the crystallographic axis c is brought into approximate 

 parallelism with the vertical spider line, and a brownish pink tint when perpendicular 

 to it. As the outlines of the earlier-formed minerals are rounded, apparently by 

 resorption, the forms of the hypersthene are irregular and serpentine, surrounding 

 and entering the curvilinear angles, embayments and interspaces that remained to 

 be occupied. 



Olivine in small rounded grains is sparsely sprinkled in the diallage. 



Three sections. 



Age. Cabotian. 



Remark.. This rock is illustrated by Wadsworth, from one of the sections 

 examined above (Bulletin ii, pages 91, 92, plate II, figure 2), but by him the included 

 mineral was considered enstatite, and the including mineral diallage. A section of 

 the hypersthene cut so as to present the axis n e , showing the prismatic and piuacoidal 

 cleavages, also shows, by the quick disappeai'ance of the hyperbolas on rotation, and 

 their tardy nature, that this orthorhombic pyroxene has the greater angle of the 

 optic axes toward the basal pinacoid, a fact which distinguishes it from enstatite. 



The diallage is fresh and original, not due to weathering nor decay, having its 

 diallagic lamellation perpendicular to the optic plane, /. c., parallel to 100. (Compare 

 No. 300.) 



It is hard to explain the remarkable freshness of this and other similar rocks, 

 already noted, since they consist of some of the most easily desti-uctible rock-forming 



