10 
POPULAR SCIENCE REVIEW. 
line, nosean, leucite, hornblende, augite, and magnetite. Long 
needle-shaped crystals (belonites) are sometimes common in it. 
Broken sanidines are often found enclosed in a trachytic base, 
and from their presence we can gather that they were already 
formed whilst the mineral mass was in motion, and thus got 
broken. 
Orthoclase. — Orthoclase, whilst presenting nearly similar 
crystalline forms to those of sanidine, differs from it in being 
very seldom clear. It is a usual component of granites and 
porphyries, as well as of other igneous rocks. The flesh-coloured 
crystals so common in some granites belong to the orthoclase 
series. When polarised, orthoclase often shows a remarkable 
irregular, banded structure, the bands being interrupted, and 
crossed by other bands at right angles to the former, and having 
a grating-like appearance. In some cases these are seen with a 
high power to be caused by an infinity of cells or tubes pene- 
trating the crystal in two directions, coincident with the planes 
of cleavage ; sometimes this banding will be in one direction 
only, but wavy. The bands frequently give the appearance of 
twin striping, and have also been supposed to be owing to an 
intergrowth of orthoclase and albite, or perthite. Quartz is 
often found enclosed in orthoclase, and occasionally a net-like 
intergrowth of these two minerals may be observed. On account 
of the general opacity of orthoclase, it is difficult to recognise 
the various microlitic minerals contained in it. It is said that 
gas cavities have never been seen in the orthoclase of granites, 
although fluid enclosures have sometimes been found. Minute 
crystals of specular iron in regular layers are abundant in some 
specimens, and often give a red tint to orthoclase and perthite. 
Triclinic Felspar . — The triclinic felspars are known at once 
under the microscope by their striped appearance. Parallel 
bands of colour, which change with each half turn of the 
analyser to the complementary tint, make this mineral to be 
one of the most beautiful of objects in the polariscope. They 
are equally well seen in plagioclase and in labradorite, and 
might well suggest some gorgeous patterns to designers of 
striped fabrics. Without the polariscope these bands may be 
faintly seen as fine dark lines in the crystal. This appearance 
is owing to the twin formation of these felspars, and as no other 
mineral forms such polysynthetic crystals, this is an unfailing 
test of its presence. Microlites of augite and haematite are not 
unfrequent in some specimens, as well as cavities. Water cavi- 
ties are found in the plagioclase of the gabbros and hypersthenic 
rocks. 
Twin Crystals . — Before passing to the next mineral, it will 
be well, perhaps, to notice here the peculiarity of twin forma- 
tions so splendidly seen in plagioclase. It is not always possible 
