MINERALOGY. lOI 



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 lamina?, 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 PI. 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 

 case 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. i on PI. 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 laminae, 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 laminoe 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 laminae becomes dark, and in which the 

 other set becomes dark, there are a few degrees of difference. This 

 causes the laminae to assume different colors in any position between 

 the Nicols. This variability of the axes in monoclinic crystals is not at 

 all uncommon. 



