W. Cross — Igneous Rocks in Wyoming. 125 



between other grains, and the leucites are almost free from 

 inclusions. 



The amphibole seems to be the last mineral to form, and it 

 varies in development. In the angular spaces between the 

 saniclines the yellowish amphibole occurs exactly as does augite 

 in the ophitic diabases, while in the leucitic areas the same 

 amphibole is developed in stout prismatic anhedra enclosing 

 the leucites, just as aegirine or segirine-augite does the nephe- 

 lites in many phonolites. In occasional spots and adjacent to 

 the pores of the rock the minerals are less intimately inter- 

 grown. Leucite is sometimes found included in sanidine, but 

 more frequently the separation is very sharp. 



There are thus in this rock two kinds of micropoikilitic 

 structure, a curious separation of the analogous silicates, leu- 

 cite and sanidine, and a porphyritic structure through the 

 prominence of phlogopite leaves. 



Sanidine. — The glassy feldspar of this rock was mentioned 

 by Kemp, who noted a rare development of simple twinning 

 and an extinction reaching a maximum of 10° from the length 

 axis. The rude crystal form renders accurate orientation 

 difficult, but since my own observations agree with Kemp's as 

 regards the angle of extinction, and as it is always the axis a of 

 elasticity which lies near the longer axis of the crystal, it 

 seems proper to interpret these prisms as developed parallel to 

 the clinoaxis. As the terminal faces of such crystals are com- 

 monly domatic planes, the prevalent rectangular outlines are 

 explained. The twinning observed must be after the Carlsbad 

 law, as extinction is parallel to the tw T inning plane. 



No microscopic intergrowths with other feldspars have been 

 seen, a natural consequence of the low soda contents of the 

 magma. Cleavage is seldom well marked, a fact which may 

 be due in part to the multitude of microlitic inclusions, serv- 

 ing to hold the cleavage plates together as in the mica of the 

 Pilot Butte rock (p. 128). 



Amphibole. — Kemp does not mention the presence of amphi- 

 bole in the rocks described by him, which are otherwise like 

 the orendite. It is indeed very variably developed in my sec- 

 tions, some 40 in number, and is not often seen in prisms well 

 adapted for study. In its optical characters this amphibole is 

 unlike any of which I can find record. Its general determina- 

 tion as an amphibole rests upon the strong cleavage parallel to 

 a prism of about 124° and the observed habit of the mineral. 



While usually irregular in outline, a few cross-sections have 

 been found which agree with amphibole in the angles of the 

 prism, cleavage, and existence of pinacoidal planes. The ter- 

 minal planes are very rarely developed, and then seem to be 

 pyramid and dome. 



