W. Cross — Igneous Rocks in Wyoming. 139 



objection applies with somewhat lessened force to the other 

 name. If a nepheline-sanidine rock is to be called phonolite, 

 an independent name is also appropriate and desirable for 

 analogous leucite-sanidine rocks. As Washington remarks, a 

 leucite-phonolite should be a leucite-nepheline-sanidine rock. 

 And it seems to me that compound names of this character 

 should always be used for the mineralogical varieties of a given 

 species. 



Madupite may be defined as consisting essentially of diop- 

 side and a magnesia-potash mica with leucite in decidedly 

 subordinate amount. Its magma was low in silica, alumina 

 and iron, rich in potash, and contained so much lime and mag- 

 nesia that silicates of these bases are the principal constituents, 

 yet controlled in their development by the strong potash 

 element. The calculation of the analysis of madupite from 

 Pilot Butte shows so clearly what must have been the products 

 of its crystallization that this rock may be considered the 

 vitrophyric equivalent of the type so denned. 



As to the systematic relationships of these rocks, there is not 

 very much to be said beyond what has already been presented 

 in discussing their relationship to each other. No other rocks 

 known to me approach very near to the types described. As 

 leucitic rocks their nearest allies are some of the Italian u leu- 

 cite-trachytes," in which, however, soda plays a more important 

 role. As pyroxene-mica rocks the relation to the minettes and 

 vogesites is most striking. In fact I think that the rocks may 

 be effectively characterized as surface equivalents of lampro- 

 phyres containing leucite instead of feldspar, rich in potash, 

 lime and magnesia, and poor in alumina and iron. It is to be 

 noted that leucite-bearing camptonites and tinguaites are now 

 known. 



It is not possible to say with certainty what mineralogical 

 composition the deep-seated portions of these magmas may 

 have. As the recent investigations of Doelter* have clearly 

 shown, the influence of physical conditions and of accompanying 

 mineralizing agents, such as fluorine, is very great in just such 

 magmas as those of the rocks under discussion. The Leucite 

 Hills magma has very possibly yielded a sanidine rock in depth, 

 yet the Boar's Tusk conduit, where exposed, is occupied by a 

 leucite rock. It is difficult to see how the madupite magma, 

 consolidated in the lower parts of its eruptive channel, can fail 

 to contain much leucite, and if only that portion of the geolog- 

 ical body were known, there can be little doubt that the rock 

 would be called a lamprophyre by the German school of petrog- 

 raphers. If coarsely granular, it might be nearly related to 

 missourite, as already pointed out. 



*C. Doelter : Synthentische Studien ; Neues Jahrbuch fur mineralogie, etc. 

 1897, Bd. i , p. 1. 



