Absaroka Range, Wyoming Territory. 45 



At first glance the most striking feature of the rock is the 

 number and size of the grains of fresh olivine scattered through 

 it. Indeed, it was this feature which first attracted attention, 

 the amount of olivine present being exceptional for the rocks 

 of this region. Associated with the olivine and the only other 

 mineral easily recognized by the eye, are large and well devel- 

 oped porphyritic augites. The two porphyritic minerals, 

 olivine and augite, lie in a groundmass made up for the most 

 part of leucite and orthoclase, very little plagioclase being pres- 

 ent. The other minerals present are magnetite, apatite, and 

 sparingly, minute flakes of brown mica The groundmass may 

 contain a small amount of glass base, but the rock comes very 

 near being holocrystalline in structure. Thin sections show no 

 indication of either nepheline or nosean. 



I am indebted to Mr. Joseph F Iddings for the following 

 observations on the leucite : 



" The leucite is partly idiomorphic and exhibits the character- 

 istic outlines derived from the icositetrahedron ; much of it, 

 however, is allotriomorphic. Many of the leucites exhibit 

 twin lamination and optical anomalies characteristic of this 

 mineral, but others appear to be wholly isotropic in thin sec- 

 tion. A number of the individuals, including those with allo- 

 triomorphic forms, carry minute augite grains arranged cen- 

 trally or in a spherical zone about the centre of the crystal. 

 They are also penetrated by needles of apatite. In parts of the 

 rock the leucites show signs of alteration to a cloudy isotropic 

 substance which is probably analcite. The rock contains rem- 

 nants of glass base which resemble those found in many basalts. 

 They are filled with brown mica and apatite and appear to 

 have been devitrified as they are no longer isotropic." 



]STo rock with which I am acquainted quite agrees with this 

 one in mineral composition, apparently failing to fall into place 

 under any generally accepted classification. It corresponds 

 more closely to a leucite^honolite than anything else ; that is 

 to say, a rock composed of orthoclase, leucite and augite. 

 According to Rosenbusch, leucite-phonolite only carries olivine 

 as an accessory mineral, whereas the Ishawooa rock is strongly 

 characterized by olivine. The presence of a considerable 

 amount of orthoclase and the small amount of plagioclase 

 seems to exclude the rock from the basalts. Provisionally 

 the rock may be called an olivine-leucite-phonolite. Mr. J. E. 

 "Whitfield of the chemical laboratory of the U. S. Geological 

 Survey, made an analysis of the rock, which is found in column 

 1 of the following table : 



