Page:The Journal of geology (IA journalofgeology11893univers).pdf/277

This page has been proofread, but needs to be validated.
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 crossing 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 lamellæ holding the inclusions polarize with a slightly different

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

color from that of the inclusion-free lamellæ. 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 lamellæ 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 Western 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.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.