TWIN, OR COMPOUND ORYSTALS. 57 
Fig. 1 represents a threeling, and 2, 3, and 4, twolings. In 3 
and 4 the combined crystals are simply in contact along the 
plane of junction ; in 2 they cross one another; the former are 
called contact-twins and the latter penetration-twins. 
Besides the above, there are also geniculated crystals, as in 
the annexed figure of a crystal of rutile. The bending has here 
taken place at equal distances from the centre 
of tae crystal, and it must therefore have been 
subsequent in time to the commencement of 
the crystal. The prism began from a simple 
molecule ; but after attaining a certain length 
an abrupt change of direction took place. The 
angle of geniculation is constant in the same 
- mineral species, for the same reason that the 
interfacial angles of planes are fixed; and it 1s such that a cross 
section directly through the geniculation is parallel to the posi- 
tion of a common secondary piane. In the figure given the 
plane of geniculation is parallel to one of the terminal edges. 
In rutile the geniculated crystals sometimes repeat the bendings 
at each end until the extremities meet to form a wheel-like 
twin. 
In some species, as albite, the reversion of position on which 
this kind of twin depends, takes place at so short intenvals that 
the crystal consists of parallel plates, 
8. 9. each plate often less than a twen- 
tieth of an inch in thickness. A sec- 
tion of such a crystal, made trans- 
verse to the plate,is given in fig. 8; 
without the twinning the section 
would have been as in fig. 9. The 
plates, as the figure shows, make with 
one another at their edges a re-enter- 
ing angle (in albite an angle of 172° 
48’), and hence a plane of the albite 
crystal at right angles to the twin- 
ning direction, is covered with a series of ridges and depressions 
which are so minute as to be only fine striations, sometimes 
requiring a magnifying power to distinguish. Such striations 
in albite are therefore an indication of the compound struc- 
ture. 
This kind of twinning is owing to successive changes of 
polarity in- the molecules as the enlargement of the crystal 
went forward. It occurs in all the triclinic feldspars, and is 
means of distinguishing them from orthoclase. 

