129 
is larger for green than for red light. — The egirine-augite 
has not infrequently been converted into an aggregate of fine 
scales of brown biotite. 
The catophorite-like hornblende shows the characters men- 
tioned in the description of the soda-granite (p. 111); it is often 
surrounded by a dark bluish-green marginal zone resembling 
arfvedsonite. Ægirine and arfvedsonite are found in small amounts 
in most specimens of the rock; they are in rather large allo- 
triomorphic anhedra which fill the interspaces between the 
felspar crystals. Ainigmatite and brown biotite occur in the 
same manner but only in a few rock-specimens; small scales 
of brown biotite, however, are not rare in the resorption-bands 
mentioned below. 
Olivine is so abundant in some specimens of the foyaite 
that it must be regarded as one of the main components, in 
other specimens, however, it is entirely absent. It is in irreg- 
ular grains sometimes reaching a length of 3 millimeters. In 
thin sections it appears colourless or slightly yellowish. The 
grains are surrounded by a zone of black metallic iron ore 
which also occurs along the cracks of the olivine. This alter- 
ation-zone has often a spongy appearance, the iron ore being 
intermingled with some biotite and hydrous iron oxide. These 
characters indicate that the olivine is of a variety rich in iron, 
perhaps fayalite. 
Eudialyte in allotriomorphic anhedra is only seldom ob- 
served in the foyaite. In some specimens it is replaced 
by pseudomorphs consisting of an aggregate of catapleiite 
crystals. 
Sequence of crystallization. — As already mentioned the 
felspar of this rock is idiomorphic against the nepheline, and 
this in turn is idiomorphic against the dark-coloured minerals. 
The latter show rather irregular variations in their mutual re- 
lation and succession, which may be illustrated by a few ex- 
amples. 
XXXVIIL. 9 
