MINERALOGY. 101 
outline. When examined with polarized light, twin crystals are most 
easily recognized, since the two parts are very differently colored, and 
stand sharply separated from one another. 
The microscopic study of the granites reveals some peculiarities of 
interest, one of which may be noticed here. Often a grain of feldspar is 
found which consists of laminze, or parts arranged at nearly right angles to 
one another. This subject has been discussed by many writers. Zirkel 
has shown that this effect is sometimes produced by the alternations of 
pure and impure orthoclase. Again: this is the characteristic structure 
of microcline which occurs interlaminated with orthoclase, as shown by 
Des Cloizeaux, and which will be considered beyond; moreover, it is a 
well known fact that orthoclase and albite are sometimes interlami- 
nated in this way. It is to such an interlamination that I now refer. 
Occasionally in the Concord granite, crystals of orthoclase are found 
which are quite impure and ingrown with them. They are very pure white 
crystals, which are arranged in two directions at right angles to one 
another, as shown in Fig. 6 on Pl. 5, and which in polarized light show the 
finest striations and the characters of albite. These crystals are arranged 
parallel and perpendicular to the clino-diagonal, since the optical deport- 
ment indicates that this section is nearly basal. I introduce this pretty 
cage so that such a kind of interpenetration may not be confounded with 
what follows. Rosenbusch has observed some effects of this kind. 
Another kind of interlamination of orthoclase is not uncommon in our 
rocks, which is illustrated in Fig. 1 on Pl. 8. This figure represents 
a crystal of orthoclase as it is commonly seen in the sections of granite 
from Chocorua mountain, when polarized light is employed. Each crystal 
is composed of a great number of irregular laminz, all having a com- 
mon direction, and which are invisible in ordinary light. The reason 
of this is, that the elasticity axes have a different position in one set 
of laminz than they do in the other, and the two sets, therefore, do 
not become dark between crossed Nicols at the same point ; but between 
the position in which one set of lamina becomes dark, and in which the 
other set becomes dark, there are a few degrees of difference. This 
causes the lamina to assume different colors in any position between 
the Nicols. This variability of the axes in monoclinic crystals is not at 
all uncommon. 
