ost usual appearance is
that of a circular area darker in colour than the surrounding
mineral. The radii of these little disc-shaped marks when well
defined are found to be remarkably uniform, in some cases four
hundredths of a millimetre and in others three hundredths, about.
These are the measurements in biotite. In other minerals the
measurements are not quite the same as in biotite. Such minute
objects are quite invisible to the naked eye. In some rocks they
are very abundant, indeed they may be crowded together in such
numbers as to darken the colour of the mineral containing them.
They have long been a mystery to petrologists.

Close examination shows that there is always a small speck of a
foreign body at the centre of the circle, and it is often
possible to identify the nature of this central substance, small
though it be. Most generally it is found to be the mineral
zircon. Now this mineral was shown by Strutt to contain radium in
quantities much exceeding those found in ordinary rock
substances.

223

Some other mineral may occasionally form the nucleus, but we
never find any which is not known to be specially likely to
contain a radioactive substance. Another circumstance we notice.
The smaller this central nucleus the more perfect in form is the
darkened circular area surrounding it. When the circle is very
perfect and the central mineral clearly defined at its centre we
find by measurement that the radius of the darkened area is
generally 0.033 mm. It may sometimes be 0.040 mm. These are
always the measurements in biotite. In other minerals the radii
are a little different.

We see in the photograph (Pl. XXIII, lower figure), much
magnified, a halo contained in biotite. We are looking at a
region in a rock-section, the rock being ground down to such a
thickness that light freely passes through it. The biotite is in
the centre of the field. Quartz and felspar surround it. The rock
is a granite. The biotite is not all one crystal. Two crystals,
mutually inclined, are cut across. The halo extends across both
crystals, but owing to the fact that polarised light is used in
taking the photograph it appears darker in one crystal than in
the other. We see the zircon which composes the nucleus. The fine
striated appearance of the biotite is due to the cleavage of that
mineral, which is cut across in the section.

The question arises whether the darkened area surrounding the
zircon may not be due to the inf