( 153 ) 
leucite rocks of the Leucite Hills are classified (Wyomingite etc.), 
the present rock is characterized by a small Al a 0 # content, a large 
proportion of oxydes of bivalent metals, and the preponderance of 
magnesia over lime, properties which these rocks have in common 
with the lamprophyric dyke rocks. 
Since the origin of mica seems to be connected with pressure 
and pneuraatolysis, so that for this reason already it is rare in the 
matrix of effusive rocks, it would appear that the occurrence in 
large Crystals as the last product of solidification would exclude a 
character as effusive rock. That the occurrence of leucite formation 
is not confined to effusive magmas of definite chemical composition 
is proved e.g. by its presence in the intrusion of missourite *), which 
is a coarse granular rock consisting of leucite, diopside and olivine 
with iron ore and apatite. On comparing the analyses I and III 
we see how much chemical and mineralogical composition are 
allied; the missourite is the deep seated modification of the rock 
here described. 
Hogbom * 2 * * * * * ) describes a growing together of leucite and mica from 
Frascati in which the latter encloses in strongly glittering bronze- 
coloured crystals several c.m. long, pseudomorphs of leucite which are 
up to 5 c.m. in length; this is explained by a sudden origin of the 
large mica individuals, which then enclosed the already crystallized 
leucites po'ikilitically. The complete freshness of the poi'kilitical leucite 
in our rock is peculiar in connection with the instability *) of this 
mineral and the conditions of origin of the mica. 
From all that has been said above it follows, that the matrix of 
the rock here described was very probably crystallized very rapidly 
and under pressure, conditions which are realized in dykes and 
small intrusions. Among the lamprophyric dyke rocks our rock is 
allied to the monchiquites and is distinguished from the minettes not 
only by the complete absence of felspar but also by the high content 
of iron ore and the nature of the femic constituents. 
At any rate, as long as the geological character of this rock is not 
determined, it must be ranged into a group with which it agrees 
in qualitative mineralogical composition; the complete absence of 
x ) Walther H. Weed and L. V. Pirsson. Missourite, a new leucite rock from 
the Highwood Monntains, Montana. Amer. Journ. of Science 1896, II, p. 815. 
2 ) A. G. Hogbom. Uber einige Mineralverwachsungen. 8. Verwachsung von 
Leucit und Glimmer. Bull. Geol. Institution of the University of Upsala. Vol. Ill, 
p. 450. 1896—1897. 
" 8 ) A. Lacroix. Etude minSralogique des produits silicates de I’eruption du Vesuve 
(Avril 1906). Paris 1907. p. 96. 
Proceedings Royal Acad. Amsterdam. Vol. XII. 
11 
