44 C. P. SHELBNER ON FOYAITE, AN ELXOLITIC 
always intergrown with some plagioclase, which, from numerous 
measurements of the angle*, I found to be oligoclase, as determined 
by M. Des Cloizeaux. This triclinic felspar forms in one case a zonal 
frame round the orthoclase, and in polarized light the twin-striation 
can be easily recognized. Numerous interpositions of hematite, as 
well as microcrystals of biotite, apatite, and magnetite are discernible 
whenever the orthoclase is tolerably pellucid. Orthoclase pre- 
dominates largely in the coarse-grained variety of Foyaite. 
Eleolite does not occur in well-defined crystals, but shows irregular 
outlines, is colourless, transparent, and full of transversal cracks 
and fissures. In one or two cases it shows, however, distinct zonal 
structure, and is frequently associated with nosean and sodalite, 
these being imbedded in it. Though on the whole fresh and un- 
altered, there are traces of decomposition, owing to the influence of 
magnetite. It is frequently studded with microlites and inter- 
positions of hematite forming rows and bands across the length and 
breadth of the mineral; grains of magnetite, hexagonal needles of 
apatite, microcrystals of titanite, and distinctly dichroitic horn- 
blende, in extremely small but clearly-defined crystals, constitute 
the remaining interpositions ; nor are there wanting rows and chains 
of fluid-cavities, such as are generally observed in this mineral. I 
may add that I found more eleolite in the fine-grained than in the 
coarse-grained Foyaite. 
Hornblende and Augite occur in Foyaite in almost equal quantity, 
and so intergrown and associated are they with each other, that it 
would often be difficult to distinguish them were it not for their 
different deportment in polarized light, the larger prismatic angle of 
hornblende in sections vertical to the main axis, and the charac- 
teristic crystallographic combination of augite, viz. the prism with 
ortho- and clinopinakoid. Moreover, I found that, when measurable, 
the angle between the main axis and the plane of vibration in 
sections parallel to the orthopinakoid is always below 20° in the 
hornblende, and varies between 40° and 50° in augite—a result 
which agrees with the measurements of Dr. Tschermak, and con- 
stitutes an important criterion when both minerals occur in juxta- 
position. The colour of both is a rich green, with occasionally a 
brownish tinge in hornblende. The crystals, which vary consider- 
ably as to size, are generally long, columnar, six-sided in horn- 
blende, eight-sided in augite, and show, on the whole, distinct out- 
lines, though these are frequently blurred by the characteristic black 
frame traceable to decomposition. Both minerals are almost invari- 
ably associated with biotite, magnetite, and not unfrequently with 
titanite, and contain not only innumerable microlites and inter- 
positions of the accessory constituents of Foyaite, but also micro- 
crystals of their own kind; nor are there wanting twins whose seam 
comes out with admirable clearness in polarized light. But the 
most remarkable feature in connexion with hornblende and augite 
in Foyaite is their peculiar imtergrowth and their tendency to 
molecular alteration or paramorphosis, not unfrequently exhibited in 
* The angle by which the plane of vibration deviates from the twin line. 
