Pirsson — Petrography of Tripyramid Mountain. 427 



acid in the original mineral. Shreds and grains of these 

 secondary minerals are also spread through the rock fabric 

 around the original hornblendes, producing confused aggre- 

 gates which more or less conceal its original nature. 



The feldspar has been mostly changed to a colorless mineral 

 of low single and moderate double refraction, a prismatic 

 columnar aggregate in structure, with good cleavage, parallel 

 extinction, uniaxial, negative character. This must be scapo- 

 lite, a not uncommon alteration product of plagioclase. Scat- 

 tered about through this are grains and dodecahedrons of 

 a colorless, isotropic mineral, with high index of refraction 

 and without good cleavage, which is inferred to be a lime 

 garnet. 



Chemical Composition. — The alteration which this rock has 

 undergone is essentially one of molecular rearrangement, not 

 ordinary weathering. Therefore the chemical composition in 

 mass has not been materially changed, and analysis seemed 

 desirable as affording the best means for judging its original 

 nature. This has been carried out by Mr. C. J. Monahan with 

 the results given in No. II of the previous table of analyses. 

 It is considered under the discussion of the classification. 



Texture. — The general plan of the texture of this rock, 

 when it was unaltered, is shown by the divergent, elongate 

 prismoids of hornblende, intermingled with the tabular feld- 

 spars and grains of iron ore and apatite, all varying considerably 

 in dimensions. It is inequigranular seriate, multiform (tabular, 

 prismoidal and equant, subhedral) divergent fabric. It is the 

 panidiomorphic-granular structure of Rosenbusch. 



Classification. — In the quantitative system the position of 

 this rock is shown by the following calculation of its norm and 

 place : 



Norm. 

 Or.. 16-68 Sal 71-52 ^ , 



Ab 31-44 F^. = ^33 =2 ' 5 - 2 > :D0Salane 



An 21-13 



Ne 2-27 L 2-27 



Di I"."". 880 p=e^2g-=0-03=:5, Germanare 



01 8-74 



Mt 5-10 K a O- + Na 2 O -_0 1 98_ 



II 5-02 OaO' - 0>)76 -1 3-3, Andase 



% °:?J K,0' 0-30 



H,O....._056 ^=— = 0-44=4, AndoBe 



Total 100-40 



It is dosalic, perfelic, alkalicalcic, and dosodic ; its coordinates 

 are 2.5.3.4 and it is therefore andose. The original mode, 

 which cannot now be quantitatively determined, was not a nor- 



