Rocks of Noyang. 23 



in sections perpendicular to the cleavage strongly dichroic 

 in shades of colour from pale yellow to almost black. It is 

 either uniaxial, or having so small an optic-axial angle 

 as not to be distinguishable when examined in the most 

 favourable cases under the microscope, by the staurescope. 

 As a rule, it is poor in inclusions. The most frequent are 

 magnetite, but I have met with instances when it contained 

 rather numerous twinned grains of felspar, which had the 

 look of having been broken from larger crystals. Rarely I 

 have observed crystalline granules of quartz. The only other 

 inclusions to be noted are a few apatite needles lying in the 

 basal section, and a few minute colourless prismatic micro- 

 liths. 



The alteration of this mica is almost wholly to some form 

 of chlorite. Scarcely an instance has come under my notice 

 in these rocks — crystalline-granular or porphyritic — in which 

 the mica has not shown traces of chloritisation. The change 

 commences at the exterior, and extends between the cleavage 

 plates towards the centre, and in some cases certain folia are 

 more attacked than others. It may be said generally of all 

 these rocks that their decomposition commences with the 

 alteration of the dark-coloured iron-magnesia-mica, which is 

 so common as to be characteristic of the whole group. 



In order to determine the nature of this mica, I separated 

 sufficient for examination from portions of the Noyang rock. 

 It is jetty black in colour, with a somewhat vitreous lustre; 

 rather difficultly separable into thin laminse, and somewhat 

 brittle, except in the thinnest flakes. Before the blowpipe 

 it fuses rather easily, and becomes magnetic. In warm 

 hydrochloric acid, it decomposes somewhat easily, the silica 

 separating as white scales. The specific gravity I found to 

 be unexpectedly low, viz., 28 1. The quantitative analysis 

 yielded the following results : — 



