36 Mr. H. C. Sorby on Slaty Cleavage. 



The best method of ascertaining whether a hmestone possesses 

 any crystalHne polarity is to examine a thin section of it with a 

 polariscope, employing a magnifying power of only a few dia- 

 meters. If the analyser be so arranged that no light passes 

 through it, and if a section of a crystal of calcareous spar, cut in 

 any direction but that perpendicular to its principal axis, be 

 placed on a stage, so constructed that it may be rotated round 

 the beam of light, it will be seen that in two positions at right 

 angles to each other it has no effect on the polai'ized light, and 

 the field remains dark. This is when one of its two axes of no 

 double refraction coincides with the plane of polarization of the 

 light; but, on rotating it from these positions, its depolarizing 

 action gradually increases, and more and more light passes 

 through the analyser, until the axes are inclined at 45° to it, and 

 then its intensity gradually diminishes until the other axis coin- 

 cides with it. The light, thus passing through the analyser, 

 has the appearance of illuminating the crystal, so that it appears 

 to become dark or light as it is rotated; being white or vai'iously 

 coloured according to its thickness and the inclination of the 

 section to the principal axis. If there be a simple, single crystal, 

 these effects take place simultaneously throughout the whole 

 surface of the section ; and, in a similar manner, if there was a 

 number of small detached crystals, all arranged on a piece of 

 glass with their crystalline axes in the same direction, they would 

 on rotation all appear dark and light at the same time. If, how- 

 ever, they were arranged promiscuously, so that their axes were 

 not inclined more in one line than in any othei", when rotated, 

 the general effect would i-emain the same in every position; 

 whilst if there was any excess, even if small, it would easily be 

 recognized. By placing then a section of limestone on the stage 

 of the polariscope and rotating it, we can thus determine with 

 very great facility whether it has any general crystalline polarity 

 in any particular direction. In this manner it may be seen that 

 the veins of calcareous spar, filling joint cracks in limestones, 

 have such an excess of polarity as shows that the crystals have 

 been formed from their sides, and that many organic fragments 

 have a polarity related to their original form and structure; but 

 when we examine a section of a cleaved limestone, cut perpendi- 

 cular to the cleavage, containing crystals of calcareous spar, do- 

 lomite, or brown-spar, it is most clearly seen that it possesses 

 no crystalline polarity whatever, or so very little as not to be 

 recognizable even by so delicate a test. I consider this as com- 

 plete a disproof as could possibly be desired of the supposition 

 that there is any relation between their slaty cleavage and cry- 

 stalline polarity. 



But it may be supposed that crystalline action might cause 



