MINERALOGY. 8/ 



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 laminae 

 I, 2, 3, and No. 2 revolved 180° about an axis at right angles to the edge 

 ii, and then No. 3 revolved in the same way about No. 2, bringing it into 

 its original position again, as illustrated in Fig. 8 a. 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 laminae, they would not in the other set in which they occupy the 

 reversed position; and that in tvv^o consecutive laminae 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 laminae 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 PI. 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 laminae is dark, which, in a basal section of oligoclase, happens 

 when the plane of the laminae 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 laminae. 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 Aunales de Chemie et de Physique, 5th series, vol. iv, 1875, and vol. ix, 1876. 



