PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 499 



Gtabbro,] 



slightly different color from the rest of the mineral and also of somewhat higher 

 index of refraction. They may represent bands of augite intergrown with the 

 orthorhombic pyroxene. 



One section examined. 



A</e. Cabotian. 



ni'imn-kx. This specimen represents one variety of the rocks to which the name 

 ' muscovado " was applied in the Fifteenth and Sixteenth Annual Reports of this 

 survey. This variety of " muscovado " is clearly a fine-grained gabbro. Rocks of 

 similar nature are developed along the northern edge of the great gabbro mass 

 and in general are of somewhat older date than the gabbro itself. (See description of 

 the Akeley Lake plate, vol. iv.) Dr. Wadsworth regarded the present mineralogical 

 composition and texture of this rock as secondary rather than primary,* but we see 

 no good reasons for considering the minerals and the texture of the rock as anything 

 but original. u. s. o. 



No. 699. GABBRO. (Altered.) 



North shore of Mavhew lake, associated with the titanic magnetite of that locality. 



/.'</. Annual Report, x, pages 81, 82; Bulletin ii, page 93, plate III, figure 2; Bulletin vi, pages 13C, 4-21. 



Gray, coarse, with reflecting surfaces of magnetite and some mica. 



Mir. The feldspar in a section 010, gives extinction on cleavage at 32. It is 

 somewhat shattered and alteration has begun cotemporary with the entrance of 

 ralcite, quartz and mica. Some of it appears to have been orthoclase. 



1't/i'o.rciii' is changed entirely and lost. In one section it is preserved in the 

 form of il in 1 1 a iji'. 



0/ir/iH' is converted into an isotropic but translucent substance, the seams in 

 which are filled with hematite. 



< 'i/l rile is rather abundant, forming a cement for the other minerals. 



Mica, probably biotite, is visible, as occasional hexagonal crystals, and as minute 

 scales distributed through the altered plagioclase. It is largely converted to 



I'/- u in ne, which, in transmitted light, is light green. Between crossed nicols it 

 is isotropic when parallel to the leaves, and blue when oblique or perpendicular to 

 them. The fibres extinguish parallel with their elongation, and are always positive 

 in elongation. This mineral is rather abundant. It presents the characteristic 

 aureoles, and constitutes here the first instance yet discovered in which the polar- 

 ization color is distinctly blue in a section whose thickness is less than .03 millimeter. 

 This blue color, however, according to Michel Levy (Mineraux des Roches, page 168), 

 is not due to the polarization proper of the mineral, but to the fact that sub-micro- 

 scopic lamellae, crossing each other, imperfectly compensate themselves, reducing 

 the resultant to a violet lilac, or gray of the first order. 



*Jlulli-tin ii, p. 94. 



