and other Rocks from Montana. 297 



The absence of titanite is remarkable, but as only one speci- 

 men has been examined it is not unlikely that this mineral as 

 well as some of the rarer minerals frequently connected with 

 such rocks may be found upon closer examinations of more 

 abundant material. 



The analysis agrees in general with that of many nepheline- 

 syenites, but differs from them all in the fact of containing 

 much more K 2 than Na 2 ; it has also great similarity with 

 Prof. Brogger's augite-syenite (Laurvikite) except as to the 

 alkalies. Most of ail it resembles the nepheline-syenite from 

 the Cape Verde Islands,* the analysis of which is copied in 

 the right column of the table (VI). The latter, however, con- 

 tains much less K 2 0. From the ordinary syenite it differs in 

 the amount of silica, as well as in the low percentage of CaO 

 and MgO. 



Only one sodalite-syenite appears to have been described 

 previously ; it occurs at Kangerdluarsuk, Greenland, and its 

 minerals have been described in detail by Mr. Joh. Lorenzen,f 

 but I have been unable to find an analysis of the rock. It 

 consists of microcline, arfvedsonite and sodalite, while nephe- 

 line is only found in places. 



I would finally like to call attention to the striking similarity 

 of the analysis of this rock with those of certain leucitophyres 

 from Rocca Monfina.:}: Under different conditions the same 

 magma, now crystallized as a sodalite-syenite, might have pro- 

 duced a leucite-feldspar rock. 



Resume. — The following rocks are described in this paper : 



1) Porphy rites and quartz-porphy rites from the Moccasin 

 Mountains. They are intrusive, holocrystalline rocks of post- 

 Cretaceous age and consist of quartz, orthoclase, soda-lime 

 feldspar and hornblende. 



2) Porphyritic, dark colored post-Cretaceous dike rocks from 

 the Bear Paw Mountains. They consist of augite, olivine, 

 biotite and triclinic feldspar and have a general resemblance to 

 the lamprophyric dike rocks of Prof. Rosenbusch. 



3) A post-Cretaceous sodalite-syenite from Square Butte. 

 This rock consists of hornblende, orthoclase, albite, sodalite 

 and analcite. No nepheline is found but the interstitial anal- 

 cite, of which there is 3 per cent, might possibly have been 

 derived from such a mineral. It contains 8 per cent of 

 sodalite. 



Washington, D. C, Dec, 1892. 



* C. Doelter. Die Vulcane der Cap Verden und ihre Producte ; cited in Arkansas 

 Geol. Survey, 1890, vol. ii, p. 81. 



\ Mineralogical Magazine, vol. v, p. 49, 1882. 



% Analyses cited in " The Origin of Igneous Rocks," by J. P. Iddings. Phil. Soc. 

 Washington, Bull., vol. xii, p. 199. 



