357 
pointed out, has failed to account for the differences between 
the individual rocks of the agpaitic complex. In that case the 
difficulties raised were the very close chemical and mineralo- 
gical relations between the individual sheets, the absence of 
contact phenomena, the gradual transitions, the horizontal distri- 
bution, and the fact that no dykes are found which are exactly 
like any of the individual agpaitic sheets; and all these factors 
tell against the assumption that differentiation occurred before 
intrusion. The regular order of succession of the banded ka- 
kortokites, and the fact that each white sheet is much thicker 
than the immediately underlying red or black sheets, are fur- 
ther difficulties which must be faced by the theory of succes- 
sive intrusion. If only white and black sheets took part in the 
building up of the complex it might be conceivable — though 
not probable considering what has already been said — that 
the black sheets had been intruded along partings between the 
white ones or vice-versa, but when the intercalation of red 
sheets below and not above each of the white ones is con- 
sidered, a conception of this kind is precluded. In short, it 
may be said that no observations whatever have been made 
which would suggest the hypothesis of successive intrusion by 
different magmas, and the only significance of the hypothesis in 
the present case is that it would refer the differentiation pro- 
cesses to unknown depths, and to causes which are beyond any 
observation, whilst it gives no explanation at all to the actual 
present arrangement of the rocks. 
The main factors which can produce differentiation in 
place are thought to be fractional crystallization and separation 
by gravity. Upon closer examination it appears improbable that 
fractional crystallization has played an important part in the 
formation of the banded kakortokites. True, fractional crystal- 
lization may be able to produce sheeted structures, and if with 
this we combine the assumption that the magma at the con- 
tact with the crystallizing rock became supersaturated with 
