138 W. Cross — Igneous Rooks in Wyoming. 



the preponderance of potash has controlled the character of 

 the ferromagnesian-lime silicates. Such rocks must be con- 

 trasted with the tinguaites, derived from magmas rich in soda, 

 producing nepheline and alkali-feldspar, with pyroxenes or 

 amphiboles of characteristics due to the entrance of soda and 

 ferric oxide into the molecules. 



The three rocks described belong to a series whose magmas 

 were relatively so rich in potash that soda has not played any 

 perceptible role in the products of crystallization. It has been 

 prevented from combining with lime in plagioclase or with 

 ferric oxide in the segirine molecule. If it does, in certain 

 rocks, go with sulphuric acid into noselite, it still fails to make 

 itself noticeable. 



Wyomingite is essentially composed of leucite, a magnesia- 

 potash mica, and diopside, all in large quantities. Its magma 

 was characterized, as has been pointed out, by richness in 

 potash with low alumina and considerable amounts of magnesia 

 and lime. Should it be demonstrated by future experience 

 that other leucite rocks actually contain more than enough 

 silica to have made sanidine in place of all the leucite, it may 

 be desirable to restrict the type to such acid leucite rocks ; but 

 it seems to me at present better to disregard this excess, in 

 definition, as quite anomalous, for the rock does not derive any 

 observable physical characteristic from the superabundant 

 silica. 



The structure of this original wyomingite is rudely fluidal 

 and porphyritic, but nevertheless of an intermediate, more or 

 less confused character, best expressed, among existing terms, 

 as hypidiomorphic. Geological occurrence is omitted from 

 these definitions because it has to my mind no legitimate place 

 in the purely petrographical classification of igneous rocks. 

 The rock is known in surface masses and in a volcanic conduit 

 near the surface. It is probable that the structure observed 

 may extend to considerable depths. Its granular equivalent 

 should receive another name. 



Orendite was derived from the same magma as the wyo- 

 mingite. It has sanidine and leucite in about equal quantities, 

 with magnesia-potash mica and diopside as the other essential 

 constituents. The development of a peculiar amphibole in the 

 type of the Leucite Hills can only be regarded as a local char- 

 acteristic. Orendite has in this present case a still greater 

 complexity in structure than the wyomingite, but much of it 

 must be considered as of local importance only. According to 

 the nomenclature of Zirkel, the rock would be classed with the 

 leucite-trachytes, and by that of Rosenbusch as leucite-phono- 

 lite. ^ While agreeing with Dr. H. S. Washington* in his 

 criticism of the term leucite-phonolite, I think that the same 



* Italian Petrological Studies, I ; Journal of Geology, vol. iv, 1896, p. 555. 



