140 
Analcime very commonly fills narrow interstices between 
the other minerals. The interstitial analcime is of the same 
habit as that occurring as an alteration product of the light- 
coloured minerals, and both kinds of analcime often belong to 
the same crystallographic individuals. For this reason the in- 
terstitial analcime is not reckoned among the primary consti- 
tuents of the rock. It is, however, decidedly older than the 
natrolite. 
Sequence of crystallization. — From the above description 
it will be seen that the order of crystallization of the main 
components is as follows: first felspar, next nepheline, and fin- 
ally ægirine and arfvedsonite. The sodalite shows a very wide 
range of crystallization, since one portion of it is older, and 
another portion of it later, than felspar. Eudialyte as a rule 
is later than felspar and older than nepheline. This order of 
crystallization is of interest because the naujaite, though closely 
related to the sodalite-foyaite, shows a different order. Only 
the place of the ægirine and the arfvedsonite at the end of the 
succession is common to both rocks and also to many other 
nepheline-syenites of related composition. This feature which 
is opposed to the general rule, that the sequence of crystalli- 
zation corresponds with decreasing basicity, has been noted 
long ago in the descriptions of ægirine- and arfvedsonite- 
bearing nepheline-syenites from several localities’. It is true 
that the crystallization of the dark-coloured minerals in the 
sodalite-foyaite may be said to have commenced at a very early 
stage of the consolidation process because minute needles of 
ægirine and arfvedsonite: are enclosed within the felspar, but 
this portion of the dark coloured minerals is quantitatively quite 
insignificant as compared with the large anhedra; and since in- 
dividuals of intermediate size are relatively rare there is no 
doubt that the bulk of the ægirine and of the arfvedsonite has 
1 V. HACKMAN, Petrographische Beschreibung des Nephelinsyenites von 
Umptek. Tennia XI, No. 2, p. 101 and 131 (1894). 
