PROWERSOSE FROM TWO BUTTES, COLORADO 171 



are the peculiar leucite rocks of the Leucite Hills, Wyoming, oren- 

 dite and wyomingite. These are, however, so extremely rich in 

 potash that they fall within the peralkaHc rang of the gallares and 

 in its perpotassic subrang, orendase. Hence they are farther removed 

 from prowersose than are other rocks of which analyses are given 

 in the table. 



The rock of Washington's tables within the rang kilauase which 

 comes nearest to prowersose, belongs to the sodipotassic subrang 

 lamarose. It is a leucite- absarokite of Yellowstone Park, but in 

 the low total of alkalis (K^O 3.79 per cent. NaaO 1.88 per cent.) 

 and in the high magnesia contents (15.96 per cent.) this rock differs 

 so markedly from the prowersose that further comparison seems 

 unnecessary. 



On the whole, the nearest relatives of the prowersose described 

 are two rocks of the Itahan province, belonging to the correspond- 

 ing subrang of the Dosalane class — namely, ciminose (II, 5, 2, 2), 

 These rocks, while notably richer in the albite molecule, maintain 

 a strong preponderance of orthoclase. 



Another rock with which comparison is particularly interesting 

 is the unique jergusose (II, 6, 2. 2), described by Pirsson from the 

 Highwood Mountains. This rock is very near the prowersose of 

 Two Buttes in normative orthoclase, albite, and anorthite, but it 

 is so rich in normative nephelite as to be brought within the lendo- 

 fehc order (6) of the Dosalane class. The close chemical relation- 

 ship evident in the analyses is made plain systematically by observ- 

 ing that, if somewhat less than 4 per cent, of normative nephelite 

 of this Jergusose were replaced by any femic molecule, it would 

 be changed to prowersose. It may be noted that there is as yet 

 no known rock of the Salfemane class corresponding to Jergusose 

 in its chemical relations. 



Modal characteristics of the rocks compared. — The prowersose 

 of Two Buttes clearly belongs to one of the rock groups illustrating 

 the fact, not yet sufficiently recognized among petrographers, that 

 there may be important variations in the mode or actual mineral 

 composition of igneous rocks possessing very similar chemical com- 

 position. The most notable variation in this particular group is 

 in the development of biotite on the one side, and leucite (or ortho- 



