PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 479 



Diabase.] 



No. 664. DIABASE. (Coarse.} 



"A little more than two miles from Horseshoe bay (north shore of lake Superior)." Evidently from 

 McParland's trail, and probably near the southwest corner of sec. 4, T. 62-4 E. 



Ref. Annual Report, x, pages 70, 71, 140; American Association for the Advancement of Science, vol. xxx, 

 page 163; Bulletin ii, pages 97, 98. 



Meg. Hand specimen not found. 



Mic. M. E. Wadsworth's description of this section is as follows:* 



" Has a section composed of brownish augite dissected by divergent feldspars and 

 containing magnetite and some secondary products. As a rule the augite is a clear 

 brown or yellowish brown, containing rows of magnetite, vapor cavities, and other 

 inclusions arranged along fissures. It also. shows in places the fine parallel cleavage 

 of diallage. This is usually towards the edges or in altered portions of the crystal. 

 Yet these points are of very minor importance compared with augite proper, which 

 occupies about two-thirds of the section. The feldspar is plagioclase and in some 

 places shows kaolinization. Both the pyroxene and the feldspar are traversed by 

 numerous fissures which are bordered by yellowish and brownish ferruginous stain- 

 ings. A serpentinous material forms brownish patches which may possibly be 

 pseudomorphs after olivine, but it, with apatite, appears oftener to be formed by the 

 alteration of the original interstitial base of rock. 



" The section is traversed in one portion by a brownish and greenish vein of 

 serpentinous material. 



"The magnetite is either of foreign origin or was the earliest mineral to 

 crystallize. This is followed by the feldspar, and lastly by the augite; the interstital 

 base being left an uncrystallized and easy altering material. In structure and 

 character there is no reason this rock should not be called a gabbro, except that the 

 pyroxene is essentially augite. The structure is decidedly granitoid. " 



The plagioclase shows abundant twinnings, and approximately equal extinc- 

 tion angles in sections cut nearly normal to 010 are as high as 31. Two sections 

 cut normal to a gave extinction angles of 61 and 60 respectively. The feldspar is 

 thus clearly near to labradorite of the composition Ab! An^ 



There is chance for a difference of opinion in regard to whether that which is 

 spoken of above as the interstitial base is really such ; it seems possible that it is 

 decomposed augite. 



The texture of the rock is markedly ophitic, the augite occurring in the large 

 plates which inclose the feldspars. The name diabase is therefore especially 

 appropriate. 



One section examined. 



Age. Cabotian. u. s. o. 



*Bulletin ii, pp. 97, 98. 



