406 Pirsson — Petrography of Tripyramid Mountain. 



ular fabric ; fracture rather crumbly ; luster dull and slightly 

 earthy, showing incipient alteration. 



Microscopic. — The study of the thin sections proves the fol- 

 lowing minerals to be present : apatite, iron ore, zircon, horn- 

 blende, augite, biotite, labradorite, microperthite and quartz. 

 The iron ore, in rather sparing grains, and the apatite, in 

 small, relatively long prisms, present nothing unusual. The 

 zircon, in rather short, thick prisms, bounded by 100 and 110, 

 shows an unusually distinct cleavage parallel to 110, and the 

 grains, though rare, are rather large. 



Augite was observed in only two cases ; in one it was seen 

 as a group of minute prismoids in one of the feldspars ; these 

 were well bounded in the prism zone by 100, 110, and 010; 

 were of a pale yellow, had an extinction angle of 40°, and were 

 twinned on 100. In the other case it formed the core of a 

 hornblende crystal, as described below. 



The hornblende is the most interesting mineral in the sec- 

 tion. It is an alkalic variety, occurring in short, thick, not 

 very well-defined prisms. It consists mostly of a brown kind 

 with the following properties, conc = 25° ; a — c = - 018 ; c, 

 yellow-brown ; b, yellow-brown ; a, pale ocher-yellow, and 

 absorption c>b>a. This is frequently capped by masses and 

 fibrous tufts of a greenish blue variety, in which c>c = 18° : 

 birefringence low, not over 0"010, and c, strong blue-green, a, very 

 pale greenish gray. These quantities are of course not exact ; 

 they represent approximations which are 

 the average from a number of sections. 

 The relations of the two hornblendes to 

 one another, and to angite, were well 

 shown in a single crystal, a drawing of 

 which is given in the adjoining figure, No. 

 1. It consists of a core of colorless augite, 

 around which the brown hornblende has 

 grown in parallel position ; at the top and 

 bottom are cappings of the bluish green 

 hornblende terminating above in acicular 

 tuf tings ; there are also some enclosed ore 

 grains. Examination of this section in 

 convergent light shows that it is not cut 

 parallel to 010 but between that and 110, 

 since the trace of an axial hyperbola 

 crosses the field. The scheme of absorp- 

 tion, with the strongest color in c, is like 

 that of barkevikite ; the mineral is unlike 

 it in the very wide extinction angle, while 

 on the other hand this character agrees 

 with Brogger's kataphorite, whose absorp- 

 It is thus intermediate between the two 



Fig. 1. 



Fig. 1. Intergrown 

 hornblende and pjrox- 



tion is strongest in b. 



