SYENITE OCCURRING IN PORTUGAL. 45 
beautifully developed crystals in which an eight-sided augite forms the 
nucleus of hornblende (PI. II. fig. 2). The tendency in augite to in- 
tergrowth has been pointed out by Dr. Tschermak, who found augite as 
nucleus not only of hornblende, but also of diallage crystals; and, simi- 
larly, the mineral known as uralite has been recognized as augite whose 
nucleus has remained intact whilst the outer portion of the crystal 
has been altered into hornblende, frequently bearing traces of its 
augitic origin. The larger crystals of hornblende and augite in 
Foyaite are often fractured: in the fine-grained variety, which 
contains considerably more of both than the coarse-grained, they 
are diffused throughout the rock, partly in fragments, now and then 
in fresh clearly-defined microcrystals, or, again, in large aggregations 
with magnetite and biotite. 
Nosean and Sodalite occur in Foyaite almost invariably associated 
and intergrown, a fact which, so far as I know, has not hitherto 
been observed in other rocks. Four- and six-sided sections of nosean, 
often showing zonal structure with a dark nucleus, are imbedded in 
and surrounded by an irregular broad belt of sodalite (Pl. II. fig. 1). 
The outlines of nosean are often indistinct, owing to the rapid de- 
composition of the mineral, whose sections are generally filled with 
the characteristic muddy yellowish dust, grains of magnetite, and 
innumerable bluish-green microcrystals ; nor are needles of apatite 
less numerous. Sodalite has similar interpositions, but is generally 
clear and transparent, with vitreous lustre and occasional zeolitic 
veins, conspicuous by their vivid polarization. Both nosean and 
sodalite are met with imbedded in eleolite ; and the frequent inter- 
growth of those two isomorphous minerals exhibits in many sections 
a very characteristic appearance. One section, in which both 
minerals are thus associated and have suffered comparatively little 
from decomposition, gave rise to a micro-chemical test for sulphuric 
acid in nosean and for chlorine in sodalite. Treated with hydro- 
chloric acid and chloride of barium, nosean gave a distinct white 
precipitate; and an unmistakable chlorine reaction resulted from 
treating sodalite with nitric acid and nitrate of silver; so that no 
doubt could be entertained as to the true nature of these two 
minerals. The crystals of nosean vary greatly in size, but measure 
sometimes as much as 1°75 millim. across the section. 
Biotite is met with in numerous brown, lamellar, transparent 
plates and flakes, showing marked dichroism and strong absorption 
in polarized light. It is invariably associated with hornblende, 
augite, and magnetite, and contains a great variety of interpositions 
of apatite, titanite, and other accessories occurring in the rock. 
The prismatic plates are frequently distorted, torn, and fractured, 
and vary greatly in size. In the fine-grained variety of Foyaite, 
biotite forms large aggregations with hornblende, augite, and 
magnetite. Muscovite occurs also in Foyaite, though very rarely, 
and only in yery small, transparent, colourless, irregular, polygonal 
plates, which are distinctly lamellar, and on that account cannot be 
mistaken for quartz, though their vivid chromatic polarization is 
liable to mislead. It should be noticed that in Foyaite this white 
