Professor R. 8. Jevons — Scratches on Minemk. 83 



not quite extinguished. When closely examined with a low power 

 it will be seen to be irregularly speckled with small spots and 

 patches of light, generally oval or elongated in shape, which often 

 give its surface a rough or shagreened appearance. These spots 

 are still visible if the section be turned to complete extincHon, and 

 are then cut in two longitudinally by a dark line. If the section 

 be further turned to the other side of the position of extinction they 

 resume their original shape, but are seen to have changed their 

 position slightly. 



The cause of this want of uniformity in extinction is easily 

 ascertained by examination with a high power. Wherever these 

 light spots occur the cleavage lines are seen to be bent laterally 

 from their normal course into a V-shape, doubtless by the scratching 

 action of the coarser grains of the polishing material during the 

 final stage in the grinding of the section. The V's piled upon one 

 another simulate a cross section of a pointed anticline or syncline of 

 great vertical range. 



It is evident, therefore, that when the section as a whole is at its 

 position of extinction, the mica along both arms of the V lies in 

 such directions that light will get through it. The dark line before 

 mentioned as cutting the spots longitudinally when the section is 

 in this position passes through the apices of the V's, and is due to 

 the fact that between their two arms the mica must lie for a short 

 distance in its original direction, now the direction of extinction. 



The spots are brightest, however, when the section as a whole is 

 not quite extinguished, for then one arm of the V is completely 

 extinguished, whilst the other is well on towards its position of 

 maximum illumination. On turning the biotite from one side to 

 a similar position on the other side of its direction of extinction 

 the e£fect is similar, but reversed ; what was previously the darker 

 arm of the V has become the lighter, and vice versa. The same 

 efifect is visible in less marked degree with only one nicol in use, on 

 account of the strong pleochroism of biotite. The angular displace- 

 ment of the arms of the V is sufficient distinctly to alter the amount 

 of absorption. 



The appearance described is presented, in my experience, only by 

 such minerals as are soft, and have a very good cleavage. It is 

 visible in biotite, muscovite, and talc, and a few other soft minerals 

 rare in rock sections. Practically, every section of these minerals 

 which I have examined shows the effect, excepting those parallel to 

 the cleavage, and I have never seen it in hornblende or any member 

 of the pyroxene or amphibole family. 



If the attention of the beginner has once been called to these 

 scratches, he never mistakes hornblende for biotite ; and only under 

 the rarest circumstances connected with the preparation of the 

 section could he mistake biotite for hornblende, even if relying 

 solely on this test. To the more advanced student the appearance 

 may occasionally prove of service in the identification of minerals, as 

 evidence of their softness. 



