258 HENRY S. WASHINGTON 
rocks. He also notes the fact that leucite occurs more often in asso- 
ciation with plagioclase than with orthoclase. 
The characteristic occurrence of leucite in effusive rocks, and: its 
absence from intrusive ones, have long been known, though recent 
observations have shown some undoubted exceptions to this latter 
statement. ‘This general association is commonly attributed to the 
greater pressure under which the intrusive rocks have consolidated, 
and also in part to the presence of mineralizers, under which condi- 
tions leucite does not seem to be stable.t The possibility of potash 
entering the biotite molecule in combination with magnesia and 
ferrous oxide under certain circumstances, while under others this 
complex molecule does not form, but its constituents crystallize as 
leucite and olivine, has been pointed out and discussed by Lemberg,? 
Backstrém,3 and others. 
The general relations between the occurrence of leucite and the 
chemical characters of the rocks have been discussed by the authors 
mentioned above, as well as by others, among whom may be men- 
tioned Iddings+ and Lacroix.s These earlier discussions were 
largely qualitative, but in a more recent paper® Iddings discusses the 
chemico-mineral relationships of rocks from a mathematical stand- 
point, illustrated by diagrams, and much of the subsequent discussion 
is closely analogous to his. In these papers of Iddings the theoretical 
limits of some ideal magmas are pointed out, and the relationships 
of actual rocks to them are considered. Still more recently? Michel 
Lévy discusses mathematically the limits of the various magmatic 
divisions of the quantitative classification, with application to certain 
rocks, but without discussion of leucite. 
t Cf. J. Lemberg, Zeitschrift der deutschen geologischen Gesellschaft, Vol. XL 
(1888), p. 635. 
2 J. Lemberg, Joc. cit., p. 636. 
3H. Backstrém, Geologiska Féreningens Forhandlingar, Vol. XVIII (1896) 
p- 155: 
4J. P. Iddings, Bulletin of the Philosophical Society of Washington, Vol. XII 
(1892), pp. 166, 176. 
5 A. Lacroix, Enclaves des roches (Macon, 1893), p. 637. 
6]. P. Iddings, Journal of Geology, Vol. VI (1898), p. 219. 
7 Michel Lévy, Bulletin de ta Carte géologique de la France, No. 92 (1903). 
