343 
as the tavite is- characterized by a poikilitic structure more ana- 
logous to that of the naujaite. 
The leucocratic agpaites are represented at Ilimausak by 
the naujaite, which after the lujavrite is the major constituent 
of the batholite. The chemical composition of the naujaite dif- 
fers so much from that of the melanocratic agpaites, that it 
might seem somewhat arbitrary to class both kinds of rocks 
into one subgroup merely on account of the alkali excess. The 
chemical type represented in the naujaite of Ilimausak has, 
however, hitherto always been found associated with melano- 
eratic agpaites (in Greenland, Kola, Transvaal, Los Islands) and 
it is probable, therefore, that the two types are genetically con- 
nected. 
Mean composition of agpaites at Ilimausak. — Since the 
variations of chemical composition within individual rock bodies 
are only imperfectly known from a small number of ana- 
lyses, the mean composition of the types cannot be accurately 
calculated. Still greater uncertainty necessarily affects any cal- 
culation of the mean composition of the entire agpaitic rock 
body, in which the volumes of the individual rock bodies 
can only be roughly estimated. The compositions given in the 
table on the next page can only, therefore, be regarded as 
probable mean compositions, and in the last column (‘Agp’) the 
calculation based on an estimation of the relative original 
volumes of the lujavrites, naujaites, etc. is given, for the pur- 
pose of affording a probable guess as to the mean composition 
of the agpattic magma at Ilimausak. 
Considering that the agpaites are chemically characterized 
by medium silica, extremely high sodium and iron, relatively 
low aluminium, and no magnesium, their composition seems to 
indicate a silicate mixture which had a definite melting point. 
Hence, if the agpaitic magma owes its origin to any kind of 
differentiation processes connected with partial solidification, 
this magma is probably a residual magma. This view is in 
