572 MISS M. I. GAEDINER ON CONTACT-ALTERATION 



A similar rock occurs on a hill to the west of Knocknairliiig, 

 where another bed of shale, probably that marked A on the map, 

 is cut across b}' the granite. 



A microscopical section of the shale B of the map, taken from 

 about 5 mile from the junction (PI. XXIII. fig. 4), shows the spaces 

 mentioned above filled in with quartz. The black colouring matter, 

 although it is still thickly scattered through the rock, has collected 

 more thickly down one slope of each wave, so that the section is 

 crossed by transverse black lines. The rock consists of very 

 minute grains, probably, from their colours of polarization, quartz 

 aiid white mica. It contains also larger flakes of mica, almost 

 colourless with vibrations parallel to their length, and a light brown 

 with transverse vibrations. These micas occur in the greatest 

 quantity on the edges of the quartz bands, and in independent 

 eyes and bands. 



Sections from the more altered bands, nearer the granite, show 

 that the colouring matter has become still more collected in lines 

 and granules. One slide shows clear vein-quartz alternating with 

 thick bands of black colouring matter and a little mica. Another 

 looks as though the minute quartz-grains and mica-flakes of the 

 little-altered shale had segregated into separate patches, and here 

 and there the small mica-flakes had built up an almost perfect 

 larger flake. A third section from a dark grey rock, with a good 

 deal of white mica, is figured in fig. 5, PI. XXIII. lEr. Teall has 

 kindly looked at this for me ; and finds that the lighter oval 

 patches, of which the rock principally consists, are biaxial. He is 

 of opinion that the mineral is probably a felspar allied to albite. 

 The grains are too small and too full of inclusions to yield cleavage- 

 flakes, by which their nature could be determined with greater 

 certainty. Occasionally one of these rounded patches contains two 

 or three white mica-flakes. These flakes have a dappled look, as 

 indeed much of the white mica has throughout, as though each large 

 flake consisted of many very small ones turned in slightly diflerent 

 directions. 



In a slide of the light-coloured granular rock, which is the final 

 stage of alteration (fig. 1, PI. XXIII.), what first strikes the eye is 

 the great quantity of white mica, in well-formed flakes, about 

 •015 inch long. A highly dichroic light-brown mica is also present, 

 often intercrystallized with the white in such a way that the two 

 extinguish together. Such intergrowths may be seen at the top 

 and on the left of the flgure. The dark mica is indicated by shading. 

 The brown mica commonly contains spots, not visible with vibra- 

 tions at right angles to the cleavage, which appear much darker 

 than the rest, and of a greenish tinge with vibrations in the oppo- 

 site direction. There are often two or three small black grains, 

 probably the colouring matter of the rock, in these spots. 



The principal part of the rock consists of granules which polarize 

 in grey tints. Though most of them seem to be quartz, biaxial 

 grains, probably of felspar, are by no means rare. The smaller 

 grains are rounded or sub-hexagonal, but the larger, as seen in the 



