410 Pirsson — Petrography of Tripyramid Mountain. 



tabular, white, sometimes pale pink, l-5 ,nm in dimensions, 

 generally dull and lusterless ; hornblende blackish-brown, 

 pitchy looking, altered ; fabric apparently equigranular ; frac- 

 ture rather crumbly ; rock partly decomposed ; weathers pale 

 brown. Except for the alteration of the ferromagnesian 

 minerals the rock resembles strikingly in color, granularity, and 

 fabric, including the grouping of the ferromagnesian minerals, 

 a specimen of feldspathic monzonite from Monzoni, which I 

 owe to the kindness of Dr. H. S. Washington. 



Microscopic. — Under the microscope the following minerals 

 are disclosed : Andesine, orthoclase, and hornblende essential ; 

 iron ore, zircon, allanite, apatite, augite, biotite, and quartz 

 accessory; with chlorite, epidote, muscovite, kaolin, and limonite 

 as alteration products. 



Of the feldspars, the andesine has a pronounced tabular 

 development, yielding elongated sections which commonly 

 show both Carlsbad and albite twinning, the latter with very 

 thin lamellae. Measurements showed it to have the composi- 

 tion Ab 3 An 6 . These feldspars, which attain a length of 5 mm , 

 are rather thickly scattered in a divergent manner through the 

 section, and are filled in between, and surrounded by, broader 

 fields, or formless masses, of orthoclase, in the manner common 

 in many monzonites. The orthoclase shows no perflate inter- 

 growth with albite ; it is considerably kaolinized, but not uni- 

 formly so; the andesine does not show this but contains in 

 places much sericitic white mica, and this is also sometimes 

 seen in the orthoclase. 



The hornblende is almost entirely changed to masses of 

 chlorite and grains of epidote, stained to a greater or lesser 

 extent with limonite, but a few unchanged pieces were found 

 which prove it to have been a brownish green variety of common 

 hornblende, with pleochroism between that color and colorless. 

 It surrounds cores of a colorless augite, or accompanies it. 



Iron ore appears occasionally in rather large grains, often 

 associated with apatite / the latter is rather abundant and the 

 stout crystals are sometimes 0"5 mm long. Zircon is also abund- 

 ant, in short thick crystals up to Oo mm in length, well devel- 

 oped and having the forms m(110) and^(lll). One crystal of 

 allanite was seen surrounded by epidote. A few shreds of 

 brown biotite in the chlorite point to a former greater abund- 

 ance of this mineral. The quarts appears here and there, asso 

 ciated with the orthoclase ; as usual, it is the last mineral to 

 crystallize, and a poor, but yet distinct, tendency to micro- 

 graphic fabric shows that it is original and not secondary from 

 alteration. Its total amount is small. 



Mode. — The rock is too altered and rather too coarse-grained 

 to yield an accurate ratio of the relative quantities of the com- 



