PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 91 



Gabbro.] 



No. 1C. GABBRO. ("Rice Point Granite.") 



Duluth. From the rock which has been slightly explored for iron ore. Belongs to the same rock as No. 1, 

 but from a point further east on the hill. 



Ref. Same as for No. 1. Also Annual Report, x, page 41. 



Meg. This is a fresh rock, of higher specific gravity than No. 1, grayish-black, 

 medium grained, having distinct ophitic structure, in some places largely made 

 up of magnetite, in the mass of which sometimes may be seen rectangular 

 surfaces. 



Mic. The coloration of all the transparent grains in a thick section, between 

 crossed nicols, is much more brilliant than that of No. 1, and the ophitic relations of 

 the pyroxene and the plagioclase plates are very marked. 



r f he feldspar exhibits Carlsbad and albite twinning, and occasionally also the 

 pericline bands. A section approximately parallel to 010 gave extinction on the edge 

 (001) (010) at 21|; another parallel to 001, gave an extinction of 22, which indicate 

 (iil-ft<'/orit<\ approximating bi/townite. 



The pijroxenic element possesses special interest, as it is usually a lamellated 

 diallage, with bright and contrasting polarization colors. The prismatic cleavage, in 

 conjunction with that parallel to the face 100, is common. The fibrous disintegration 

 which precedes the change to amphibole is always perpendicular to the last mentioned 

 cleavage. There is a great range in the degree of change manifest in the diallage. In 

 the section examined there is no apparent tendency to amphibole, but to chlorite; 

 at least no amphibole colors nor cleavage is visible. Some of the diallages are nearly 

 intact, and others are entirely changed. Those which are yet intact embrace 

 magnetite in grains of considerable size, which sometimes show angular forms, as if 

 original, while those which are changed have fine granular aggregations of magnetite 

 of irregular shapes at their centres, the periphery of the diallages being made up of 

 a rim of opaque veriditic matter, apparently of chlorite. The magnetites so embraced 

 in decayed grains of diallage are frequently impacted in epidote, or epidote and 

 chlorite. In the unaltered or slightly altered diallages the contact between the 

 diallage and the included magnetites is clear and abrupt, indicating the original 

 nature of the magnetite. See figure 3, showing cleavages of diallage. 



The arrangement of the magnetite in some of the principal magnetic grains is 

 in lines (as seen in section) or sheets. It suggests that the lamellation of the diallage 

 parallel to 100 may have determined its position, and hence, on the view above that 

 the magnetite in the main is not a magmatic secretion, but is a secondary generation 

 after consolidation, before cooling, the lamellation of the diallage must have had a 

 very early origin, prior to the genesis of this secondary magnetite, and hence was 

 probably original, and not a result of change from augite. Still, it is possible, if not 

 probable, that an earlier stage of the diallage was augitic, and that both the 



