W. Cross — Igneous Rocks in Wyoming. 121 



inn. 



delicate play of colors. The largest leaves are only 2 mm or 3 

 in diameter, and while some are of microscopic size, none be- 

 long properly to the groundmass, which consists of leucite and 

 diopside, with but small amounts of apatite and other variable 

 elements to be mentioned. 



The mica is of remarkably weak absorption and pleochroism, 

 the latter ranging only from a pale salmon-pink to pale yellow. 

 Basal sections show the exit of a negative bisectrix and an 

 optic angle which is large for mica, reaching about 35° accord- 

 ing to a measurement made by Prof. L. V. Pirsson, who has 

 described a very similar mica in one of the leucite rocks of 

 Montana. Sections normal to the base often exhibit a poly- 

 synthic basal twinning, and by this means one can readily 

 establish the fact of a measurable angle, reaching as much as 

 3°, between a and 6. Inclusions, excepting a few of glass, are 

 very rare in these mica plates, and pure material for chemical 

 analysis was procured with the Thoulet solution by W. F. 

 Hillebrand, who analyzed it with the result to be given later 

 on. From the analysis it is plain that this mica is a phlogopite, 

 and as far as I can ascertain it is the first definitely known 

 occurrence of this variety in true igneous rocks. Both Zirkel 

 and Kemp have alluded to the peculiar character of this mica, 

 which they called biotite. 



The groundmass holding the phlogopite crystals is largely 

 made up of leucite, with many very small pale-green or color- 

 less microlites of diopside between them. In all the sections I 

 have examined the number of leucites with distinct crystal 

 planes is very small compared with the multitude of minute 

 roundish anhedra. Those individuals with icositetrahedral 

 form are slightly larger than the anhedra, and they are further 

 distinguished by a zone of minute inclusions and often by re- 

 entering angles on the planes of the crystal, this being the 

 nearest observed approach to a skeleton development. The 

 roundish grains usually vary between -01 mm and -05 mm in diam- 

 eter. JNo trace of double refraction has been seen. 



A variability in the development of leucite in different 

 places is indicated by the description of Zirkel, who refers to 

 the sharp crystal form of each of the minute individuals. From 

 the comparatively low amount of sulphuric acid in the rock, it 

 seems that but little noselite can be assumed to be present, 

 although it must be developed quite abundantly in the similar 

 rock of the Boar's Tusk, and is there indistinguishable from 

 leucite. 



The microlites of the typical wyomingite are for the most 

 part too minute for certain identification except by comparison 

 with the few larger ones and with the very distinct diopside of 

 other rocks to be described. From the amount of phosphoric 



