128 W. Cross — Igneous Rocks in Wyoming. 



eter, holding the diopside microlites and perofskite grains in as 

 great numbers as does the residual glass. The streams of diop- 

 side prisms pass without any change through the phlogopite 

 grains, and a very pronounced form of the micropoikilitic 

 structure is thus produced. The anhedra of phlogopite have 

 very uneven, ragged outer borders, and show none of the usual 

 tendency to develop in plates parallel to the cleavage, and this 

 parting is here much less strongly developed than in any other 

 occurrence of mica known to me. The cleavage lines appear 

 in this case like those in feldspar or amphibole, being sharp and 

 clear, but by no means so numerous as is usual. 



That this peculiarly developed mineral is mica seems to me 

 sufficiently established by the following data: The color is 

 almost identical with that of the phlogopite. Sections showing 

 strongest cleavage polarize very brilliantly and extinguish par- 

 allel to the cleavage, as far as can be ascertained. The bril- 

 liant polarization of most sections is very striking, but a few 

 may be found where the maximum color is the pale-blue of 

 melilite. Such sections show in convergent light the exit of a 

 negative bisectrix, the optical angle being large. The plane of 

 the optical axes is parallel to a sharp boundary line which was 

 observed in one case. The pleochroism is even fainter than in 

 the mica of the leucite rocks, but by comparing the sections 

 showing best cleavage with those exhibiting the optical iigure 

 the following optical scheme was made out : 



a yellow, with pinkish tinge occasionally, lies normal to the 



cleavage, i. e. near c. 

 h pink, parallel to b. 

 c straw yellow, nearly parallel to a. 



This orientation is the same as that of the phlogopite in the 

 leucite rocks. 



Perofskite and magnetite are developed in minute crystals 

 of O02 mm to 0*03 mm , and the former is much the more abundant. 

 The grains referred to perofskite are roundish crystals of very 

 high index of refraction, yellow-brown color, isotropic, and 

 only those included in the phlogopite are entirely fresh, the 

 rest showing a dull white cloudy alteration product which 

 causes them to stand out in marked contrast to the magnetites 

 when the section is viewed by reflected light. Some very 

 minute needles of apparently yellowish color may be seen in 

 the phlogopite, with a high power. These seem most plausibly 

 referred to rutile, though no regular arrangement was observed. 



The glass base of the rock is isotropic but not clear and 

 transparent, being clouded by indistinct globulites and micro- 

 lites which seem to have a yellowish-green color, or at least 



