Pirsson — Petrography of Tripyramid Mountain. 423 



The plagioelase which composes the groundmass is much 

 smaller, averaging 0*03 to O^OS" 1 ™ in greatest dimensions. It is 

 developed in rather thick tablets which tend to nearly square 

 outlines on b (010) and which commonly exhibit both Carlsbad 

 and albite twinning. The crystals are usually zonal in struc- 

 ture, having a distinct, uniform, interior core consisting of 

 Ab,An„ which passes into phases much richer in soda at the 

 outer margins. As shown by the calculations from the analysis, 

 the average plagioelase present is approximately Ab 3 An 2 . 



A moderate amount of orthoclase is also present in grains of 

 abo\it the same size as those of plagioelase and it frequently 

 shows Carlsbad twins. 



In the angles between the feldspars there is seen, here and 

 there, minute areas of a colorless substance which sometimes 

 polarizes faintly, and sometimes does not. In some cases the 

 latter sections yield a faint uniaxial negative cross. This is 

 inferred to be nepheliie and on testing with nitric acid the 

 powdered rock was found to yield a definite, though not large, 

 amount of gelatinous silica. Part of this must come from the 

 olivine, but the amount seemed too large to be wholly derived 

 from the small quantity of that mineral known to be present, 

 and was thus held to be also indicative of nephelite. It is also 

 possible that, in part, this interstitial material may be sodalite, as 

 indicated somewhat by the distinct reaction for chlorine which 

 the rock yields, though in this connection the possibility of a 

 chlor-apatite must be considered. It may also, in part, be 

 aualcite, and in one or two cases this seemed possible, as a fairly 

 good cubic cleavage was observed. At all events, a very small 

 quantity of a feldspathoid is present, either wholly, or in part, 

 nephelite, and this is also indicated in the calculation of the 

 norm. 



Mode. — The mode of this rock is a normative one and the 

 quantities of the minerals present are those shown in the cal- 

 culated norm, the only essential deviation being that a small 

 quantity of the calculated orthoclase and olivine are really 

 represented by the two or three per cent of biotite which the 

 rock contains. 



Texture. — The fabric which this rock presents in the section 

 is like that shown by many gabbros, but on a minute scale. It 

 is produced by the short divergent feldspar laths, scattered 

 through which are the grains and masses of augite, iron ore and 

 olivine. A somewhat similar fabric is seen in diabases when they 

 become deficient in augite, and lose the characteristic ophitic 

 texture, and in some basalts. It appears in some lamprophyres, 

 as in certain kersantites where biotite replaces augite, and in 

 spessartites with hornblende. It is a quite different type of 

 texture from the equigranular one observed in some dikes 



