416 Pirsson — Petrography of Tripyramid Mountain. 



2-3'"'" on the c (001) cleavage; the latter clearly striated by the 

 albite twinning ; rarely the feldspar shows a blue opalescence ; 

 occasional grains of dark greenish ferromagnesian minerals, 

 black ones of iron ore, and glittering, bronzy specks of biotite 

 are also seen. Resembles anorthosite from many Canadian 

 and Adirondack occurrences. Weathers with a dark gray 

 crust. 



Microscopic. — In thin sections the following minerals are 

 found to be present ; labradorite feldspar, pyroxene, olivine, 

 iron ore, biotite, apatite. 



The feldspar is in the form of flat tables parallel to o ( 010) : 

 it is unaltered and clear, save for swarms of slender dark 

 microlites orientated parallel to three different systems ; they 

 appear as lines, dashes and dots. In places these microlites 

 are somewhat larger, and it can then be seen that they are 

 sometimes birefringent, have an index of refraction greater 

 than the feldspar, and sometimes change in their length for 

 short distances into black opaque bodies. They appear like 

 the inclusions of a similar character found in the feldspars of 

 gabbros and anorthosites from other localities, but their exact 

 nature could not be determined. The feldspar is twinned 

 according to the albite and pericline laws, but rarely according 

 to the Carlsbad. It is sometimes zonally built and the opti- 

 cal properties show in addition a feature sometimes, though 

 rarely, seen in that one albite lamella has a different rela- 

 tion of the Ab and An molecules which compose it from 

 another twin lamella lying beside it. Such feldspars have been 

 described by Michel Levy,* Federofff and the writer.^ The 

 average composition of the feldspar as determined by optical 

 methods is Ab 7 An 13 , while that reckoned from the chemical 

 analysis of the rock is Ab 2 An a — the first equals Ab 2] An 39 , the 

 second Ab 2c An 39 . This feldspar was also analyzed by E. S. 

 Dana, and while made on material too impure to yield defi- 

 nitely exact ratios, since at that time the later methods of 

 obtaining pure material were unknown, the results also suffici- 

 ently indicate it to be a basic labradorite. 



The pyroxene is of a very pale brown color ; in places filled 

 with minute microlites like the feldspar, it does not show the 

 diallagic parting parallel to a (100) ; it contains numerous grains 

 of iron ore and scales of biotite but no feldspar ; its form is 

 entirely irregular, filling spaces between the feldspars ; it shows 

 no sign of alteration ; the maximum angle of c on c was meas- 

 ured as 40°. 



*Mineraux des Eoches, p. 84, 1888. 



\ Zeitschr. fur Kryst, vol. xxiv, p. 130, 1894. 



i Bull. Geol. Soc. Amer., vol. vi, p. 412, 1895. 



