46 C. P. SHEIBNER ON FOYAITE, AN ELMOLITIC 
mica is not found intergrown with biotite, but with felspar and 
eleeolite. 
One of the most noteworthy features in Foyaite is the very 
frequent occurrence of titanite. This mineral is distinguished by 
the freshness of its crystals and the distinctness of its outlines. 
It occurs with great regularity in all the varieties of Foyaite, 
and is at once recognized by its delicate yellow colour, its 
transparency, its strong lustre, and the singular purity of its 
substance. It is met with chiefly in elongated rhombic and six- 
sided prisms, which are traceable to the favourite combination of this 
mineral, viz. the hemipyramid with positive hemidome, clinodome, 
and basal plane. Though frequently intergrown with hornblende 
and augite, it occurs more particularly in isolated crystals which are 
found imbedded in eleeolite and biotite. Some of the crystals abound 
in interpositions of hornblende, augite, biotite, apatite, and magnetite ; 
and in some rare cases the edges show some slight traces of decom- 
position. Pleochroism is generally so faint that it is hardly per- 
ceptible ; on the other hand, twins are by no means so rare in this 
titanite as might be supposed, and it is notably in crystals of 
small size that a delicate seam attests twin-structure. 
The occurrence of magnetite in opaque black grains or polygonal 
outlines is so uniform in rocks that it does not call for special 
reference here; nor need I dwellon hematite, pyrite, and apatite, which 
occur in Foyaite, as in other kinds of rock, purely as adventitious 
minerals—hematite in pink microscopic six-sided plates, or in rows 
and bands as interpositions ; pyrite in isolated opaque grains show- 
ing yellowish reflected light; and apatite, whose microscopic, 
colourless, transparent, columnar, hexagonal prisms, wedges, and 
needles are truly ubiquitous. 
As regards the chemical composition of Foyaite and its con- 
stituents, it is notably eleolite which deserves special investigation, 
with a view to ascertain the presence and percentage of sulphuric 
acid; and an analysis I made of this mineral gave the following 
result :— 
LCA IE SRR h Bee cet, 43°46 per cent. 
Sesquioxide of iron ...... Nicos eR 
Arima. ooh ane BRET. 
deine He See, REO RS 040) =<, 
Miaonesia iiur eee 0,097 78s 
Soar cr. eeaian, ements t NSPS y 
Potassa eke spy mins 434  ,, 
Sulphuric acid@se reer: OsUSe es, 
Wratten): .. array aie rics 6 ES ay 
D9: Gomes 
This result coincides with the analyses made by Prof. Scheerer, 
Rammelsberg, and others of eleolite occurring in other rocks, though 
the presence of sulphuric acid has been established only in a few 
cases. An analysis of Foyaite made by Mr. T. 8. Humpidge gave 
the following composition of the rock ;— 
