176 J. F. KEMP THE LEUCITE HILLS OF WYOMING. 



practically the same as those of the more eastern flows, it has an isotropic 

 groundmass which is apparently glass. 



PREVIOUS OBSERVATIONS BY Z IRK EL. 



The descriptions by Professor Zirkel were based on several specimens 

 gathered by Mr S. F. Emmons in the necessarily hurried reconnoissance 

 of the great area covered by the geologists of the Survey of the Fortieth 

 Parallel. They came, as the writer is informed by Mr Emmons, from 

 the large south mesa, and yielded to Professor Zirkel the following 

 results : * 



" They have a light yellowish gray, felsitic-looking, and very finely porous mass, 

 in which the only macroscopical inclusion is some brownish yellow and reddish 

 browm mica. This mica is not in six-sided or rounded plates, but in the form of 

 remarkably long stripes and dashes, such as have seldom been observed. No other 

 ingredients are visible to the naked eye, and the specimens do not disclose their 

 rich secretions of leucite. At the first glimpse of the rock under the microscope 

 the leucite appears, with its innumerable, very sharply outlined, colorless octagonal 

 sections, .035 millimeter in diameter. None of the European rocks are as rich in 

 leucite as these, and there is scarcely one in which the forms of the sections are so 

 regular and so similar. As is the rule with all such small bodies, the sections are 

 entirely dark between crossed nicols. . . . All of these leucite sections include 

 quite pale green augite grains. . . . [The inclusions are described in detail.] 

 . . . There are mixed with the leucites in this rock, as independent ingredi- 

 ents, pale green prisms, acicular needles, and microlites, which surely belong to 

 augite, although their shape is indistinct, and larger, better crystallized individ- 

 uals do not occur. In this fine aggregation and intermixture of leucite and augite 

 the large biotite stripes are imbedded, and none of them of microscopical size was 

 observed. This curiously colored mica, which resembles ormolu and whose long, 

 thin streaks appear in surprising distinctness in the light rock mass, is remarkable 

 for its comparatively very feeble absorption. . . . These plates seem for the 

 most part to be scattered through the rock with some measure of parallelism, and 

 hence the sections prepared parallel to the rock cleavage show no transverse sec- 

 tions of mica, but only basal ones. There is no trace of monoclinic or striated 

 feldspar, and hornblende, olivine, mellilite, haiiyne, and nosean are wanting. A 

 small quantity of magnetite is present, and also a considerable number of com- 

 paratively thick apatites. . . . Occasionally indistinct colorless rectangular or 

 oblong bodies appear, which possibly belong to nepheline. No corresponding hex- 

 agons are visible ; but in any case this mineral must be relatively very rare. Some 

 brownish black opaque microlites occur at intervals, and a few of them are in- 

 cluded in the mica. 



"The external aspect and the mineralogical composition of these rocks differ 

 not a little from the other leucite-bearing masses. Their unusual light-gray color 

 is produced by the extraordinary abundance of leucite and their comparative 



* Microscopical Petrography, p. 260. 



