IF. Cross — Igneous Hocks in Wyoming. 123 



rock. The matrix is a finer dust of the same origin, and it is 

 generally pale-green in color. Fragments of sandstone, lime- 

 stone, oolite, and some granular rocks rich in dark silicates 

 were observed. 



Microscopical examination reveals leucite, diopside, and 

 phlogopite as the important minerals, with a few small biotite 

 leaves and apatite in unusually large prisms, with axial inclu- 

 sions. Both leucite and diopside are developed in much more 

 distinct crystals than in the preceding rock. A few large irregu- 

 lar grains of augite and phlogopite intergrown seem probably 

 to belong to some of the rocks appearing in fragments. 



Leucite is developed in well-formed crystals which include 

 many minute diopside needles. They reach -05 mm in diameter 

 and are very abundant, exceeding all other constituents in 

 amount. Diopside occurs in short, colorless prisms, seldom 

 twinned, of hexagonal cross-section through the suppression of 

 one pinacoidal plane. 



These constituents lie in a very subordinate cloudy-gray base, 

 which obscures the leucites except in the thinnest places. By 

 high powers a faint greenish color seems visible, and as there 

 is some double refraction in the mass it appears probable that 

 there is a microlitic development of diopside and a scanty glass 

 base. No magnetite was seen in the rock. 



Orendite. 



The principal rock of the Leucite Hills, whose chief con- 

 stituents are leucite and sanidine, with phlogopite, amphi- 

 bole and diopside, seems to me worthy of a special name, and 

 it is proposed to call this rock and its equivalents elsewhere 

 orendite, after the prominent butte on the northeastern side of 

 the Hills, where it is well developed. The reasons for this 

 proposition and the scope of its suggested application are pre- 

 sented fully in a subsequent section, after discussion of the 

 chemical analyses. 



Megascopical description. — The orendite is characterized by 

 the same dull reddish-brown or gray tones seen in the wyo- 

 mingite, and its only distinct megascopic constituent is phlo- 

 gopite. All the specimens collected were subordinately vesic- 

 ular, but that is of course not an essential feature. On North 

 Table Butte the rocks are much lighter in color than w r as 

 observed elsewhere, being yellowish to straw-colored. Under 

 a lens the mass of the rock seems almost saccharoidal in tex- 

 ture, showing a white granular mass colored by a small amount 

 of indistinct pinkish or yellowish matter. 



In most cases a few dull grains of orthoclase will be seen, 

 but they are corroded and show the same character as the feld- 

 spar of larger included rock fragments to be described later. 

 These are therefore not regarded as phenocrysts properly 



