160 
offers a peculiar interest. Perthitic intergrowths are entirely 
wanting: on optical examination some of the crystals prove to 
be pure microcline while others are pure albite; both kinds 
may be found in the same rock-slices and very often lie side 
by side. In some specimens of the rock microcline-crystals 
and albite-crystals are present in about equal amounts, in others 
the former are predominant, in others again albite-crystals pre- 
vail. These variations are not connected with any kind of va- 
riation in the outer habit of the rock. 
between crossed nicols the microcline shows an irregular 
twin-structure (Fig. 15) of the same type as in the naujaite; 
not seldom one individual of the twin is in excess of the other. 
The albite exhibits the ordinary polysynthetic twinning accord- 
ing to the albite-law (Fig. 16). Fluid-cavities and minute included 
needles of ægirine are not rare. 
In almost all other rocks the alkali-felspars, as is known, 
occur either in perthitic intergrowth or in homogeneous mixed 
crystals (soda-orthoclase etc... The complete separation into 
crystals of pure microcline on the one hand and crystals of pure 
albite on the other hand, characterising the Greenlandic lujavr- 
ites!, may be supposed to result from the following conditions: 
— (1) Movements of the magma during the crystallisation are 
indicated by the structure of the rock. (2) An uncommonly 
high degree of liquidity of the magma is probable for reasons 
inferred from the chemical composition of the rock and from 
the peculiar character of the differentiation-processes which wili 
be considered more fully in the final chapter. (3) A relatively 
low temperature of consolidation probably also characterises 
the lujavrites. The latter assumption is supported by the 
Meddelelser om Gronland XIV, p. 93 (1994). — In the lujavrites of 
Kola and of Transvaal a similar behaviour of the alkali-felspars may 
sometimes be observed (W. Ramsay, Fennia XV, No. 2, p. 7, 1899; H. 
A. Brouwer, Oorsprong en samenstelling der Transwaalsche nephelien- 
syenieten, s'Gravenhage 1910, р. 120. 
