PETKOGRAPHIC GEOLOGY AND DESCRIPTIONS. 545 



Gabtro.] 



No. 786. GABBRO. (Granulitic.) 



Portage between Spotted Rock and South Side lakes, but north of the summit crossed by this portage; 

 perhaps in N. E. % sec. 23, T. 63-5 W. 



Ref. Annual Report, x, page 103; Bulletin ii, page 91. 



Meg. A fine-grained, brownish, granular rock. 



Mir. The section shows a fine-grained, granular aggregate composed essentially 

 of plagiodase, pijro.rcuc and magnetite. The pyroxene is both orthorhombic and 

 monoclinic. Biotite, sometimes in large plates, is also present. 



Two sections examined. 



Age. Cabotian. 



Remarks. This rock closely resembles No. 698. u. s. G. 



No. 787. GABBRO (with olivine). 



On same portage as No. 786, but south of the summit. 



Ref. Annual Report, x, page 103; Bulletin ii, pages 59, 90, 91, plate III, figure 1. 



Meg. A medium-grained, gray, granitic rock, composed of feldspar, pyroxene, 

 olivine and magnetite. 



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



" A grayish medium-grained rock, composed principally of a clear, glassy feldspar 

 holding brownish oHciiir. di ullage and magnetite. The section is composed of clear 

 feldspar, holding yellowish-brown oliviue, darker brown enstatite, and magnetite. 



" The feldspar shows a clear brilliant polarization, while the polysynthetic 

 twinning according to the albite-pericline law is marked even in common light. The 

 fibrous alteration has begun to appear along the cleavage planes and fissures giving 

 it, in places, the common cloudy appearance seen in feldspars of considerable age. 

 The olivine stands next in abundance to the feldspar and is much fissured with a 

 yellowish ferruginous staining along the fissures, and sometimes even extending 

 through the entire mass of the mineral. It contains some magnetite, part of which 

 is secondary, while the olivine itself appears to be foreign. 



" The enstatite is mostly built out upon the olivine grains as an apparent con- 

 tinuation of them but only rarely do they correspond in optical orientation. The 

 enstatite shows the beginning of alteration, indicated by a brownish color, and the 

 development of fine smoky ferruginous bands parallel to the principal cleavage 

 It also has an irregular cross fracture. The magnetite is often bordered by forming 

 fibrous biotite.f 



"This rock answers to the olivine-noryte of Rosenbusch, and in its structure it 

 somewhat macroscopically resembles some of the basaltic meteorites. Figure 1, 

 plate III, shows the structure of the rock." . One section examined. 



Bulletin ii, pp. 90,91. 



tlnvma: Copper- Hear ing Rocks, 1888, p. 51 ; HA WES : Litholotjij of New Hampshire , 1878, p. 205 ; WADSWORTH : Litho- 

 loyicdl Studies, 1884, p. 77. 



36 



