THE BASIC MASSIVE ROCKS, ETC. 703 



characteristic of gabbro diallage. These are often arranged in 

 straight lines crossing the parting planes. They are frequently 

 so crowded that the line of inclusions appears as a dark bar cross- 

 ing the diallage at various inclinations to the cleavage, as in the 

 most notable case (No. 8786), where the direction of the bar 

 cuts the prismatic cleavage at 21 ° and on the same side of it as 

 the extinction, which is 37 (see Fig. 2). Under polarized light 

 the diallage appears as though polysynthetically twinned. The 

 lamellae holding the inclusions polarize with a slightly different 



Fig. 2. Inclusions in Augite. Section 8786. X ca. 18. 



color from that of the inclusion-free lamellae. Moreover, the 

 material in the immediate vicinity of the several inclusions seems 

 to be more changed from its original condition than portions of 

 the same lamellae at a greater distance from them. This would 

 indicate that the inclusions have absorbed some of the material 

 of the pyroxene in their growth, and consequently that they are 

 not original inclusions, as are those found by Williams 1 in the 

 Cortlandt peridotites and norites, but are secondary like those 

 discovered by Judd 2 in the peridotites and gabbros of the West- 

 ern Islands of Scotland. 



Under high powers a second cleavage can be detected as a 

 series of fine lines perpendicular to the prismatic cleavage, in 

 sections parallel to the vertical axis. Along these cleavage lines 

 are disposed the inclusions with their long axes so arranged in 

 the direction of the lines as to suggest that the latter were planes 

 of easy solution — that the decomposition of the diallage first 

 took place along them, and then attacked the pyroxene on both 

 sides. 



1 Am. Jour. Sci., 3rd ser., vol. 31, 1886, p. 33; and vol. 33, 1887, p. 141. 

 2 Quart. Jour. Geol. Soc, London, vol. 41, 1885, p. 354. 



