THE FORMATION OF LEUCITE IN IGNEOUS ROCKS 
HENRY S. WASHINGTON 
INTRODUCTION 
In the following pages are presented the results of an attempt 
at the determination of the chemical characters of magmas which 
may control the formation of leucite in igneous rocks. Connected 
with this is an examination of the validity of the orders of affinity of 
the bases for silica and for alumina, which have been assumed as 
fundamental in the calculation of the norm in the quantitative classi- 
fication. The paper is an extension of some previous studies, to 
be referred to subsequently, undertaken under the auspices of the 
Carnegie Institution. It may be pointed out here that the methods 
employed in the case of leucite are capable of application to other 
minerals. 
I must express my deep obligations to my friends, Dr. Cross and 
Professor Iddings, with whom the first draft of this paper was dis- 
cussed, for valuable suggestions in regard to the plotting of the 
diagrams and other matters, by which the work has gained much in 
simplicity and, it is to be hoped, in intelligibility. 
It is commonly assumed or explicitly stated that the rocks in 
which leucite occurs are usually low in silica and high in alkalies, 
especially potash. But no definite limits for silica or for potash have 
been established, and leucite-bearing rocks are known which are 
quite high in silica and low in potash. Over forty years ago Roth? 
pointed out that the silica percentage of the rock could be either 
above or below that of leucite. Roth, in the last-cited work, and 
subsequently Zirkel,? observe that quartz does not occur with leu- 
cite, and the latter speaks of this as one of the few laws which can as 
yet be enunciated in regard to the mineral associations in igneous 
1J. Roth, Zeitschrift der deutschen geologischen Gesellschajt, Vol. XVI (1864), 
p- 690; and also Beitrage zur Petrographie der plutonischen Gesteine (1869), p. 90. 
2F. Zirkel, Lehrbuch (1893), Vol. I, p. 646. 
257, 
