LEUCITE HILLS. 237 



apatite. The mica is notable for its very feeble power of absorption, so 

 that when examined with one nicol its plates do not become black, but only 

 reddish-brown. To the comparatively small proportion of augite and mag- 

 netite in these rocks may be ascribed their unusually light color as com- 

 pared with the European leucite rocks. Associated with the more compact 

 rock, which has some resemblance in texture to the older European rocks, 

 is a porous variety, which gradually passes into a distinct pumice. An 

 analysis of a medium-grained, somewhat porous specimen, made by Mr. R. 

 W. Woodward, gave: 



Sihca 54.42 



Alumina 13.37 



Ferric oxide 0.61 



Ferrous oxide 3.52 



Lime 4.38 



Magnesia 6.37 



Soda-' 1.60 



Potassa 10.73 



Lithia trace 



Carbonic acid ^ 1.82 



Water 2.76 



99.58 



The pumice is a very singular-appearing rock, consisting of a spongy 

 mass of a light reddish-brown color, in which the cavities are generally 

 lengthened out in parallel directions, while through the whole rock are in- 

 numerable minute flakes of a peculiar reddish mica, which occur in elon- 

 gated plates. 



Around the Leucite Hills, the eruptive flows seem to have had some 

 slight metamorphic action on the underlying sandstones, and a bed of 

 semi-crystalline white limestone was found under them to the north, 

 which may probably represent the metamoi'phism of some of the beds 

 of the Green River Tertiary. No fragments of this eruptive rock were 

 found, except in the immediate vicinity of these hills, though the pecu- 

 liar metamornhism of the summit strata at Black Butte would seem 



