MINERALOGY. 87 
except when the elasticity axis corresponds with the plane of vibration 
of the light, which is when the vertical axis makes a certain angle with 
this plane. But, suppose the crystal to be divided up into the laminz 
I, 2, 3, and No: 2 revolved 180° about an axis at right angles to the edge 
zz, and then No. 3 revolved in the same way about No. 2, bringing it into 
its original position again, as illustrated in Fig. 8a. It is plain that the 
effect of this, if often repeated, would be in the first place to cover the 
base O with striations running parallel to the edge of the brachy-pinna- 
coid, and to bring the axes of elasticity into such positions that, when 
they corresponded with the plane of vibration of the light in one set of 
the laminz, they would not in the other set in which they occupy the 
reversed position; and that in two consecutive lamine the axes of elas- 
ticity would make double the angle with one another that they do with 
the vertical axis of the crystal; and hence in polarized light the consec- 
utive laminze would be differently colored, and the section would appear 
banded. 
The extent to which the twinning may go on is illustrated in Fig. 6 
on Pl. 7, which is drawn from a basal section of a crystal of oligoclase 
from the Antrim granite. This section is so placed in the figure that 
one set of laminz is dark, which, in a basal section of oligoclase, happens 
when the plane of the laminze makes an angle of from three to four de- 
grees with the plane of the vibration of the light. A millimetre is placed 
in the figure for comparison, and it is seen that there are forty repetitions 
of the twinning in one millimetre, or over a thousand to one inch. The 
yellow crystal to the right is orthoclase. All triclinic feldspars have 
a basal cleavage; and, as the striation is there plainly shown, they 
are sometimes called striated feldspars. In polarized light, the effect of 
twinning would be seen in all sections save those parallel to the brachy- 
pinnacoid, which is the plane of the laminz. As the basal cleavage of 
feldspar is so easy, sections large enough for microscopic examinations 
are easily obtained from sizable crystals without labor; and mineralo- 
gists are thankful to Des Cloizeaux* for giving the position which the 
axes of elasticity bear to the brachy-diagonal axis of the crystals in 
basal sections of the different feldspars, and which furnishes a most 
ready way for their determination. 
* Des Cloizeaux Annales de Chemie et de Physique, 5th series, vol. iv, 1875, and vol. ix, 1876. 
