Rocks of Noyang* 37 



rock did I find any portions of the original mineral remain- 

 ing intact. Yet after carefully observing the various stages 

 of alteration from mica to chlorite in the crystalline-granular 

 rocks of this group, I cannot feel any doubt that this also 

 is merely a secondary mineral. It occurs either in more or 

 less well-defined hexagonal or rectangular sections. The 

 former undergo no change of colour or of tint when exa- 

 mined over the polariser alone, while the latter are more or 

 less markedly dichroic. The ray which traverses the crystal 

 in the direction of the axis "c" is almost colourless, while 

 that perpendicular to it is of some shade of green. It is, 

 however, very rare to find one of these chlorite sections 

 homogeneous throughout. The basal section, when examined 

 between crossed nicols, usually shows a more or less wide 

 margin, which behaves like an isotropic or uniaxial mineral 

 when seen in the direction of the optic axis, while more or 

 less of the central part is doubly refracting. The sections 

 perpendicular to the cleavage planes show similar features ; 

 the green outside edges of the plates become wholly obscured 

 when their fibrous structure is parallel to the plane of the 

 nicol, while the colourless central parts show strong chromatic 

 polarisation, resembling that of potassa-mica, and in no 

 position are the numerous fibrous aggregates simultaneously 

 obscured, nor could I find any fibres that behaved otherwise 

 than would do those of a triclinic mineral. This suggests a 

 colourless chloritic mineral, perhaps Leuchtenbergite. In the 

 cleavage planes there have been deposited ores of iron and 

 perhaps titanium. 



In order to learn something more as to the nature of the 

 porphyritic felspars and of the chlorite pseudomorphs, I 

 digested a slice of this rock with occasional boiling in hydro- 

 chloric acid for nearly a month. On then examining it I 

 found that the hydrated iron ores had been removed from 

 the slice, leaving it altogether much clearer and more trans- 

 lucent than before. The chlorite pseudomorphs were some- 

 what bleached, and some of the folia were more attacked than 

 others, but a great part of the black amorphous substance in 

 the chlorite was unaffected, and could therefore not be 

 magnetite. 



The porphyritic felspars were not only not affected, but 

 were much brightened by the removal of minute alteration 

 products ; but there was one exception, where part of the 

 porphyritic crystal had evidently been replaced by some 

 carbonate — probably calcite — and had been totally removed, 



