140 IF. Cross — Igneous Rocks in Wyoming. 



Of course to call these surface rocks lamprophyres is to dis- 

 regard the fundamental conception of Rosenbusch as to the 

 geological significance of this group of " dike-rocks." But I 

 fully agree with Iddings,* who has discussed this question at 

 some length, that the lam pro phy res are abundantly represented 

 at the surface by lavas differing in structure and mineral ogical 

 composition from the dike rocks, as a result of differing condi- 

 tions of consolidation. The Boar's Tusk wyomingite has, more- 

 over, a decided resemblance to minette in habit, making due 

 allowance for the different roles played by leucite and sanidine 

 as a result of contrasting crystal forms. 



It is with great regret that I confess my inability to state 

 the existing relationship in occurrence between wyomingite 

 and orendite. The former is massive, the latter always vesicu- 

 lar, and I believe them to be merely different parts of one 

 flow. The pumice contains neither leucite nor sanidine. 



Inclusions in the Leucite Hills Rocks. 



There are many inclusions of foreign rocks in the lavas of 

 the Leucite Hills and in the Boar's Tusk. These were also 

 noted by Kemp, who found them especially abundant in the 

 southwestern part of the principal mesa, and who mentions 

 sandstone as the most common type. The fragments occur in 

 all parts of the Hills visited by my party, and many different 

 rocks were observed : sandstone, limestone, oolite, granite, and 

 some peculiar mineral combinations to be mentioned. 



The most noteworthy feature of these included fragments is 

 the very distinct caustic action of the lava displayed in most 

 cases. Some quartzose sandstone inclusions are vitrified in 

 considerable part, and certain granitic rocks have also suffered 

 partial fusion. It is noticeable that a rounded form is common 

 among these inclusions, but there is little or no evidence that 

 this rounding is the result of fusion. 



Rocks composed of pyroxene and plagioclase feldspar seem 

 quite abundant, and some were found consisting almost wholly 

 of plagioclase rich in lime. Still others are basic combinations 

 of augite and biotite with but little feldspathic material. 



The action of the magma upon these inclusions may be illus- 

 trated from a few instances. One small inclusion of rounded 

 form appears megascopically to be a medium-grained rock of 

 green pyroxene and feldspar, but it is noticeable that the feld- 

 spar grains are not distinct and cleavage can not be distinctly 

 made out. Microscopical examination shows the feldspar to 

 have been plagioclase, but it has been acted upon by the 

 magma and partially destroyed. Along cleavage lines and on 



* The Origin of Igneous Rocks, Bull. Phil. Soc, Washington, vol. xii, pp. 172- 

 178, 1892. 



