Pirsson — Petrography of Tripyramid Mountain. 407 



and the colors have this intermediate character. The blue- 

 green variety, which at first thought suggests arfvedsonite, 

 differs from this in that c is nearest c (18°) and the strongest 

 absorption lies in c, while in arfvedsonite these properties lie 

 in o. It appears to be in fact closely allied to the hastingsite 

 of Adams and Harrington, which has o on c = 25°, c and b deep 

 blue-green, a, yellow-green. 



The biotite is not abundant, occurring in occasional small, 

 well-formed tablets of the ordinary pleochroic brown variety. 



The feldspars consist of labradorite in small amount, associ- 

 ated with much microperthite. The former is in relatively thin 

 tables elongated parallel to 010, well twinned according to 

 albite and Carlsbad laws, and having the composition AbjAn, ; 

 these serve as cores to masses of much more highly sodic feld- 

 spar, or anorthoclase, which have grown around them in par- 

 allel orientation ; these masses as a whole have no definite 

 outward form, and they constitute the white feldspar spots 

 seen on the surface of the hand specimen. 



The microperthite is by far the most abundant mineral in 

 the rock ; it presents no unusual features, being the ordinary 

 intergrowth of orthoclase and albite ; it is somewhat altered to 

 kaolin, and this accounts for its lusterless surface in the speci- 

 men, and the half per cent of water shown by the analysis. It 

 is not intended by this to convey the idea that the rock is 

 badly altered, for such is not the case ; only that the feldspar 

 is to some degree changed. 



A small amount of quarts is present in the interstices between 

 the feldspars ; it has a distinct tendency in places to micro- 

 graphic intergrowth with them. 



The order of crystallization is the normal one, beginning 

 with apatite, zircon and iron ore, then hornblende, then labra- 

 dorite followed by microperthite, whose final stage of consoli- 

 dation was simultaneous with that of the small remnant of 

 silica as quartz. 



Mode.— The mineral composition of the rock is essentially 

 that shown by the calculated norm, and the mode is therefore 

 normative. The differences between the mode and norm con- 

 sist in that the greater part of the calculated pyroxene, with 

 some of the iron ore and a little soda of the feldspar, is present 

 as hornblende ; the hypersthene and a little orthoclase are 

 represented by biotite ; the adjustment of these differences 

 frees enough silica to convert the small quantity of normative 

 nephelite into albite and yield a little free quartz. 



The composition is then approximately : 



