412 Pirsson — Petrography of Tripyramid Mountain. 



Megascopic. — Ilolocrystalline, medium-grained, 1-3" 1 " 1 , mot- 

 tled dark and light in color, average tone, gray ; consists of 

 formless, greenish, to greyish Mack, ferromagnesian mineral 

 grains mingled with pale brownish-white feldspar ones. With 

 the lens hronze-colored cleavages of biotite are occasionally 

 seen and, on the feldspar cleavage, tine striations of albite twin- 

 ning. Weathers brown, becoming stained by ferric oxide. 



Microscopic. — The microscope discloses in thin section the 

 following component minerals ; apatite, iron ore, biotite, hyper- 

 sthene, augite, labradorite, andesine, orthoclase and quartz. 



The apatite is rather abundant for this mineral, in short to 

 long prisms. The iron ore, which is titaniferous magnetite, is 

 rather abundant, and, while sometimes showing crystal outlines, 

 is often very irregular and encloses apatite and augite. The 

 augite, in large crystals, is very irregular in form and then 

 includes iron ore, apatite, biotite, and even labradorite. Small 

 crystals are often quite automorphic, bounded in the prism 

 zone by 100, 010 and 110 equally developed; these are quite 

 free from inclusions, and often twinned on 100. The usual 

 prismatic cleavage is good, the parting 100 was observed, but 

 is not common. The color is a pale green ; nonpleochroic. 

 Maximum extinction angle 13°. 



The hypersthene is in rather long, columnar crystals, poorly 

 terminated and of good size, though occasional rounded gran- 

 ules also occur. It shows the usual parallel extinction and 

 rather low birefringence. It is very distinctly, though not 

 strongly, pleochroic, the ray parallel to c being green, while a 

 is red. The rather faint pleochroism would tend to show a 

 variety approaching bronzite in the content of iron. Along 

 cracks there is a slight alteration to serpentine. It carries 

 inclusions of apatite, iron ore, and biotite. 



The biotite, of the usual deep reddish-brown variety with 

 strong absorption, is scattered through the rock in irregular 

 flakes. It is apt to coat the iron ore, but also occurs inde- 

 pendently, and, as noted above, is found as a frequent inclu- 

 sion in the pyroxenes. Some large poikilitic crj'stals were 

 observed which included all the other constituents, even labra- 

 dorite. 



The plagioclase has the form of short, broad, book- 

 shaped masses, which yield columnar sections when parallel to 

 the basal plane, but much of it is quite irregular in shape. It 

 is quite clear, and fresh, and free from inclusions. It has both 

 albite and Carlsbad twinning ; the albite twinning also shows 

 that the crystals in many cases are curved, or bent, or even 

 faulted, pointing to movement under pressure, and, in connec- 

 tion with this point the reader is referred to the discussion on 

 the origin of Tripyramid in the preceding paper. In composi- 



