4 8 
AN INVESTIGATION INTO THE 
or less distinct parallelism of the constituent minerals, owing to movements 
during the final stages of the consolidation of the rock, representing in fact a 
sort of fluidal structure. Traces of this are seen in the specimen from 
which the colored photograph accompanying this description was taken, but 
the prism of the rock on which the elastic constants were measured, while 
otherwise identical with the specimen photographed, showed no traces 
whatsoever of the fluidal structure in question, but was absolutely massive. 
Under the microscope the rock is seen to be composed chiefly of light-colored 
"salic" constituents of which feldspar is by far the most abundant. This is 
chiefly orthoclase, but this mineral is much intergrown with plagioclase, which 
is also present in not inconsiderable amount. These feldspars have for the 
most part a lath-shaped development, and it is on account of the more or less 
parallel arrangement of these laths and of the hornblende crystals that the 
9000 
1000 
o 4-0 eo l^o leo 200 ^4o 28O 
S TRA IN 
FIG. 19. Nepheline Syenite. Stress-strain curves. 
fluidal structure above mentioned results. Associated with the feldspar is 
nepheline in rather small amount, and also nosean often in well-defined 
individuals. These occur in some cases as inclusions in the feldspar. In 
other cases they lie in the corners between the latter. The dark ("femic") 
constituents are represented chiefly by a greenish-brown alkali hornblende, 
with which biotite is associated in much smaller amount. This hornblende 
has a tendency to an acicular development. There are also present in small 
amounts, as accessory constituents, sphene magnetite and pyrite. 
The rock is fresh, there being no signs of decomposition. Although the 
nepheline and the nosean are in most cases somewhat altered, the changes 
which have overtaken them are quite independent of surface decay. 
A color-process photograph of a polished surface is seen in Plate XI A, and 
a photomicrograph taken between crossed nicols and magnified 30 diam- 
eters is shown in Plate XI B. 
