88 MINERALOGY AND LITHOLOGY. 



It is of course plain that a basal cleavage piece of orthoclase, if put on 

 the stage of the microscope between crossed Nicol prisms, would be 

 dark when the straight edge formed by the meeting of the brachy-diag- 

 onal and the basal cleavages was parallel to the plane of vibration of the 

 light, since it is monoclinic. If a section of any other feldspar is obtained 

 in the same way, and put between the crossed Nicols, it will appear as if 

 made up of a series of bands parallel to the edge of the brachy-pinna- 

 coid ; and when these bands or this edge is placed parallel to the plane 

 of vibration of the light, none of the bands will be dark, since no crys- 

 tallographic and elasticity axes coincide in triclinic crystals ; but, on 

 turning a certain number of degrees to one side, a part of them will be 

 dark, and, on turning the same, or nearly the same, to the other side, the 

 rest will be dark, while the first become again brightly colored. The 

 amount of the revolution necessary to produce these results varies with 

 the different species, and furnishes the most ready way to discriminate 

 between them. That is the angle that an elasticity axis makes with the 

 brachy-diagonal can be determined, or the angle that the section must be 

 revolved from the position, when one set of bands is dark, till the others 

 become dark, (which would be just twice as much as the first angle, and 

 is the angle between the elasticity axes in two parts of a twinned crystal) 

 may be determined. These angles for the different species of feldspar, 

 as fixed by Des Cloizeaux, are as follows : 



Oliogoclase, 

 Labradorite, 

 Albite, 

 Anorthite, . 

 Microcline, 



Fig. 6 on PI. 7 may be taken as an illustration. It is drawn from a 

 basal section of oligoclase from Antrim. In oligoclase a plane of elastic- 

 ity cuts the base, making an angle of 3° with the brachydiagonal, and 

 hence when these striae are placed parallel with the hair line indicating 

 the plane of vibration of the light none of the striae are dark, but when 

 the section is revolved three degrees one set of bands becomes black, 

 while the other takes a higher color. If now the section be revolved 



