347 
DIFFERENTIATION OF THE AGPAITIC MAGMA. 
It has been shown above that the nepheline-syenites which 
make up the bulk of the stratified Ilimausak batholite have 
certain common distinctive chemical characters, and it was 
found convenient to group them under a common name: 
agpaites (see p. 338). The several agpaites — viz. sodalite- 
foyaite, naujaite, lujavrites, and kakortokites — are closely 
related to one another, both from a chemical and from a min- 
eralogical point of view. All of them contain practically the same 
minerals, but in very different proportions. From this evidence it 
is extremely probable that all the agpaitic rocks of Ilimausak are 
genetically connected, or speaking more precisely, are differen- 
tiation products of one parent magma: the agpaitic magma. 
Further, if we consider, (1) the stratiform arrangement of the 
rocks, (2) the absence of any structural differences between 
the inner part of each stratum and its border zones, and (3) the 
gradual transitions from sheet to sheet, it will be seen that the 
conditions are such as to suggest a differentiation in situ. This 
supposition also agrees with the circumstance that the stratified 
batholite of Ilimausak is the only occurrence of agpaitic rocks 
in southern Greenland. 
In the following paragraphs we shall try to show that 
the assumption of a differentiation in place is not incon- 
sistent with the observed facts, and that in this case the 
differentiation may partly be explained on the old principle 
of mechanical separation by gravity of crystals from their 
magma. 
As is well known observations bearing directly upon a 
process of this kind are extremely rare, and we may safely 
assume that in most batholites the viscous condition of the 
magma has prevented even heavy crystals from sinking’. This, 
! A. Harker, Natural History of Igneous Rocks, 1909, p. 322. 
