180 J. F. KEMP — THE LEUCITE HILLS OF WYOMING. 



It forms small, stocky rods in the groundmass and larger micro-pheno- 

 crysts up to one millimeter in length and .25 of a millimeter in diameter. 

 It is colorless, often twinned on (100), and has the high extinctions of 

 common augite. The felt of little rods makes up the chief part of the 

 ground mass and exhibits flow phenomena of marked excellence. The 

 biotite contains many little augites. Small, rounded leucites can be de- 

 tected with high powers, but are in small amount. Much of the inter- 

 stitial filling of the felt of augites is isotropic and homogeneous and is 

 regarded as glass, although Pirsson's suggestive observations on the iso- 

 tropic groundmasses of basic rocks makes one suspicious of the presence 

 of analcite.* This, however, in lack of an analysis, does not impress one 

 as a particularly basic rock. Its color is light and its whole macroscopic 

 appearance suggests trachyte. It was described as trachyte by Mr Em- 

 mons,f but from the mineralogical details given by him it is clear that 

 there must have been some confusion of slides, probably in the prepa- 

 ration of them. In its systematic relations the rock is mineralogically 

 nearest the augitites, but it is clearly a variant from the group of rocks 

 of the Leucite hills, with which, however, it has the mica and augite in 

 common. 



Chemical Composition. 



The only analyses yet published are two, one made by R. W. Wood- 

 ward with silica and the alkalies in duplicate, as given under I and its 

 reference below, and the other b} r Pawel, made in Germany for Professor 

 Zirkel. With it are placed several other analyses of western leucitic 



rocks. 



I. II. III. IV. V. VI. VII. VIII. IX. 



Si0 2 . 54.32 56.30 54.04 52.91 51.93 46.51 47.28 52.11 55.11 



A1 2 3 13.37 12.63 20.27 19.49 20.29 11.86 11.56 23.01 16.07 



Fe 2 3 0.61 6.92 4.66 4.78 3.59 7.59 3.52 8.41 3.04 



FeO 3.52 0.64 2.05 1.20 4.39 5.71 1.75 8.46 



CaO.... 4.38 5.63 2.75 2.47 1.65 7.41 9.20 3.40 6.46 



MgO 6.37 5.08 0.16 0.29 0.22 4.73 13.17 2.18 3.10 



K 2 10.73 11.50 6.79 7.88 9.81 8.71 2.17 3.10 5.07 



Na 2 1.60 2.21 8.56 7.13 8.49 2.39 2.73 5.37 1.58 



H 2 0.. 2.76 1.93 1.19 1.09 3.55 2.96 1.10 0.89 



C0 2 1.82 none. 0.25 



99.58 100.27 99.80 98.19} 98.52} 97.14} 98.30} 100.43} 100.53 

 Sp.gr......... 2.22 , 2.838 2.546 



* The Monchiquites or Analcite Group of Igneous Rocks, Journal of Geology, vol. iv, p. 679. 



t Fortieth Parallel Survey Reports, vol. ii, p. 238. 



J Additional components are given under the references which bring up the totals. 



