"THE FELSPARS. 
219 
to observe that when they do not become dark at the same time the 
two parts do not extinguish when the dividing line makes equal angles 
on each side with the principal sections of the Nicols. 
We shall often find in these sections a very indefinite extinction — 
i.e., the whole surface does not become equally dark all over at any 
possible position of the section, but takes a curious mottled look. The 
cause of this is not certainly known, but it has been suggested that it 
is owing to the crystal being compound instead of simple, built up 
of an extremely large number of very minute ones of another of the 
group, a potash felspar, but differing from Orthoclase in some 
important particulars. 
Now we shall probably soon find among the colourless, but often 
rather cloudy grains, which we are inclined, at the first glance, to take 
for the mineral we have been studying, some which, under the 
searching test of polarised light, show themselves to be made up of a 
very large number of fine bands alternately light and dark, or if the 
section is thick enough, alternately differently coloured. On rotating 
this section between crossed prisms alternate bands will be found to 
extinguish together and at a considerable angle at times from the 
position of extinction of the other ones, and this, even when the lines 
of junction make at the extinctions equal angles with the principal 
planes of the prisms. This shows that we have to do with a mineral 
which is not symmetrical (physically) with regard to the plane of 
composition of the twinning; in fact it will be found that we have 
here a crystal of what is called the Triclinic system, in which, that is, 
the three axes are all inclined to each other, in which, therefore, no 
plane can be found which will divide the form into two exactly similar 
parts—no plane of symmetry, in fact. If the block of which I spoke 
earlier had not had any of its edges rectangular, it would be seen 
that it was impossible to find any position in which there was not an 
apparent break between the object and its reflected image when it was 
placed on the looking glass. 
The mineral which we have thus detected is one of the Triclinic or 
Plagioclase felspars, so called because if we examine the cleavages in 
a crystal of one of them we shall find that these are no longer at 
right angles to each other, but inclose an angle, measured over the 
edge in which they meet, of about 86J°, varying slightly with the 
different sorts. 
The rough models before you are attempts to give visible proof of 
some of the exterior properties of these minerals, especially in regard to 
that by which we have detected them in the rock, the multiple twinning. 
In the first place, we find that, seeing that no face of the crystal is a 
plane of symmetry, we may, by simply turning one component through 
half a circle, keeping the corresponding planes together, produce a 
twinning possible according to the laws of crystals; and, indeed, this 
is the commonest of all among the Triclinic felspars, and from its 
prevalence in Albite is called the Albite twinning. In most cases this 
is repeated very many times, and the models show that the result is 
