bo 
— 
— 
slightly inferior to that of Canada balsam, and the birefringence 
is about equal to that of felspar. Under the microscope the 
lamellar structure is seen to be due to rows of fluid-cavities; 
otherwise the mineral is perfectly transparent and fresh. It ge- 
latinizes with hydrochloric acid and on microchemical test gives 
a strong reaction for sodium. The mineral could not be chemic- 
ally analyzed for lack of material. 
The fourth kind of phenocrysts is represented by relatively 
numerous, dull, greenish-black pseudomorphs, I or 2 millimeters 
in diameter. Under the microscope they are seen to be made 
up of a fibrous green hornblende surrounded by a reaction 
rim of green biotite and egirine. The fibrous hornblende 
contains remnants of big crystals of a light grayish augite with 
large extinction angles and with numerous inclusions of apatite, 
zircon, and iron ore. 
The ground mass has a trachytoid structure. It consists 
of felspar, nepheline and decomposition products of this min- 
eral, ægirine, and green biotite. A gray augite with green 
margin and little grains of titanite are found in small quantities. 
The felspar of the ground mass is a lath-shaped microperthite, 
and there is a strong tendency to parallel arrangement of the 
laths. Their length is about 05 millimeter. A little analcime is 
often found among the felspar laths. 
Marginal zones of the dyke. — A four meters broad zone 
on each side differs from the central part of the dyke. While 
the latter abounds in phenocrysts and has an almost dense 
ground mass the marginal zones are extremely poor in phe- 
nocrysts, and their ground mass is coarser and of a foyaitic 
structure. They consist of felspar, nepheline, and ægirine. The 
felspar is a microcline-microperthite of about the same type as 
in the naujaite (p. 147); it is in small plates showing a very 
marked parallel arrangement. The nepheline is very abundant 
and less idiomorphic than the felspar; it contains numerous 
ægirine-microlites, and has partially been converted into aggre- 
