100 /. P. IDDINGS 



the most notable features of Diagram I, is the evident limita- 

 tion of the range of alkali-ratios along the curved lines. These 

 lines were suggested by the distribution of analyses in the 

 diagram and represent in one case, the upper line, the possible 

 range of analyses for rocks consisting wholly of silica, alumina, 

 and soda, the two last elements being always in the proportion 

 of i : i ; corresponding to nephelite at one extreme, succeeded 

 by a mixture of nephelite and a possible metasilicate of these 

 bases, or of nephelite and albite, and of albite and quartz, and 

 finally of quartz alone. In the case of the lower line the curve 

 corresponds to a possible range of analyses for rocks consisting 

 of silica, alumina, and potash, the latter being in the proportion 

 of i : I. The mineral range would be from pure quartz, through 

 a mixture of quartz and potash feldspar, to one of potash feld- 

 spar and leucite, to a possible orthosilicate of aluminium and 

 potassium, corresponding to a potassium nephelite. In not a 

 single instance is the sodium-aluminium limit transgressed, and 

 only a few cases occur beyond the potassium limit. The rocks 

 actually consist of the minerals just named. A nearly pure 

 nephelite rock exists as urtite from the Kola peninsula, 1 and as 

 portions of the nephelite-syenite of Dungannon township, Canada, 

 described by Professor F. D. Adams. 2 A nearly pure albite rock 

 has been found in California, by Mr. H. W. Turner. 3 



The fact that these limits are so closely approached, but 

 never transgressed, is a clear indication that the alkalis do not 

 exist in greater proportions than may satisfy this series of 

 ortho-, meta- and polysilicate molecules. There are instances, 

 however, in which part of the requisite alumina is replaced by 

 ferric oxide. 4 There are a few cases in the neighborhood of 



1 Ramsay, W.. Urtit, ein basisches Endglied der Augitsyenit-Nephelinsyenit- 

 Serie, Geol. Foren. i Stockholm Forhandl, — Bd. 18, Haft 6, 1896, p. 459. 



2 Adams, F. D., On the Occurrence of a Large Area of Nepheline-Syenite in the 

 Township of Dungannon, Ontario, Am. Jour. Sci., Vol. XLVIII, July 1894, P- I0 - 



3 Further Contributions to the Geology of the Sierra Nevada, 17th Ann. Rep. 

 U. S. Geol. Survey, Washington, 1896, p. 728. 



4 The most marked exception to this rule is the group of rocks from Leucite Hills, 

 Wyo., described by Cross, Igneous Rocks of the Leucite Hills and Pilot Butte, 

 Wyoming, Am. Jour. Sci., Vol. IV, 1897, pp. 115-141. 



