737 
rocks being erystallized almost simultaneously as well above as below 
with the lujaurites, likewise ought to show a parallel structure. 
4. In the breccia-zone between the Greenland lujaurites and the 
naujaites differentiation towards the marginal zones occurs. As the 
lujaurites between the naujaite-blocks distinguish themselves from 
the others only by the fact that the parallel arrangement of the 
composing minerals follows planes which bend round the naujaite- 
blocks, a continual current ought necessarily to have taken place also 
between the naujaite-blocks, which would have prevented the diffe- 
rentiation. 
An other explanation for the strongly varying structures we have 
described will now be suggested. 
The parent magma of all these rocks is characterized by a high 
percentage of pneumatolytie gases and connected with it a strong 
power of crystallization and a thin fluidity maintained to a compara- 
tively low temperature. As the different foyaitic rocks of Greenland 
and most likely also those of the Pilandsbergen have crystallized 
with only very slight differences in time, the temperature can only 
be a secondary factor in the mode of formation of these greatly 
varying structures. The differentiations caused by fractional crystal- 
lization or by separation according to the specific gravity may 
likewise be left out of account, as they modify chiefly the compo- 
sition and not in the first place the structure of the rocks. 
From such a magma coarse granular varieties will be formed 
under undisturbed conditions of crystallization, whilst poikilitie struc- 
tures can be explained by differences in power of crystallization, in 
connection with affinity and with the relations between the quantities 
and the solubilities of the components. For the fine granular varieties 
a rapid mode of crystallization is essential, but movements in the 
magma are not required. This rapid erystallization can be caused 
by the escape of gases from the magma which remained thin fluid 
down to a low temperature on account of the great percentage of 
pneumatolytic gases kept in solution, whereas by the escape of the 
gases it suddenly becomes viscous, so that large crystals cannot 
develop themselves any more. 
If now during the crystallization a one-sided pressure prevails, this 
pressure — transformed in the thin fluid magma into an all-sided 
one — will be able to make its influence felt. Consequently it is 
not accidental that exactly the fine-granular lujaurites show a great 
inclination to parallel-structure. /n the viscows magma no strong 
currents can take place; no parallel structure can thus be formed 
in it. In case larger feldsparcrystals had already been formed before 
