228 BULLETIN OF THE 
mon ; in some specimens the rock is dotted with small white crystals of 
sodalite. In one instance the rock holds large phenocrysts of flesh- 
colored feldspar in tabular prisms with terminal planes, and is porous 
owing to the presence of small irregular cavities, whose rough walls are 
lined with limonite. Plates of biotite are rarely seen. ‘There is a dis- 
tinct fluidal structure in the dikes and sheets which is visible in the 
hand specimens by the parallel arrangement of the phenocrysts and a 
slight fissility parallel to the plane of flow. 
Characters in Thin Sections. — Feldspar. — In the thin sections of this 
type the feldspar phenocrysts appear in long rectangular sections, or in 
broader and less regular crystals. Carlsbad *twins are as frequent as 
single crystals. A certain number of these feldspars polarize homoge- 
neously, and extinguish parallel to the length of the rectangular sec- 
tions, or nearly so, like sanidine; but in many sections a very fine 
multiple twinning is present parallel to the brachypinacoid (albite law), 
and sometimes there is present with this a faint double twinning (mi- 
crocline structure), or segments are multiply twinned in one direction, 
which is not the same in adjoining segmeuts of the feldspar. Cleavage 
sections were prepared from one of these phenocrysts with the following 
result : — 
Basal cleavage (O P) extinction 24° to 3° oblique to the trace of the 
2d cleavage (¢9 P o& ). 
Second cleavage (gg P @ ) extinction about 9° oblique to the trace of 
the basal cleavage. 
Specific gravity of the piece 2.583. 
This specimen is therefore a triclinic feldspar intermediate between 
microcline and albite (soda-microcline — anorthoclase) and the anomalous 
polarization of the feldspar sections in the rock is thus explained. These 
phenocrysts contain as inclusions apatite, sodalite, augite, egirine, and 
biotite. The egirine needles appear to have formed toward the close 
of the feldspar period, as they are commonly included in the peripheral 
portion of the crystal. 
Sodalite is present in some specimens, occurring either in crystals 
large enough to dot the rock macroscopically, or only visible in the 
section. It appears in the slides in the usual apparent hexagons or 
squares, sometimes rounded or corroded by the groundmass. The min- 
eral is colorless, with a low refractive index and isotropic, unless zeo- 
litization has occurred. It sometimes contains secondary calcite. The 
mineral gelatinizes easily with acid, and the micro-chemical tests show 
