Rocks of Noyang. 25 



are not quite those which Dr. Heddle assigns to haughtonite. 

 The Noyang mica stands, however, between biotite and 

 lepidomelane, although not equidistant to each. Its physical 

 characters and behaviour before the blowpipe and to 

 hydrochloric acid remove it from biotite, and the relatively 

 small amount of ferric oxide removes it from lepidomelane. It 

 may be, however, considered as representing a compound 

 of three-fifths biotite and two-fifths lepidomelane, and with 

 such a composition its low specific gravity is more in 

 accord. 



Although the composition of this mica does not quite agree 

 with that given by Dr. Heddle for haughtonite, it is suffi- 

 ciently near to justify me, I think, in referring it to that 

 variety of magnesia-iron-mica. 



It is convenient at this place to speak of the alteration 

 products of this mica. 



Chloritic minerals are so extremely common in these 

 crystalline-granular rocks that they may be looked upon as 

 one of the most characteristic constituents. The rocks which 

 I have examined in thin slices afford plentiful evidence of 

 the manner in which the conversion of the magnesia-iron- 

 micas into chloritic minerals has taken place. Sections of 

 the rock which I have taken for illustration afforded me all 

 stages from the merest alterations in the edges of the cleavage 

 plates to the complete conversion of the mica into a chloritic 

 pseudomorph. Chloritisation is attended by the elimina- 

 tion of ores of iron, which are deposited as magnetite either 

 in the chlorite crystals 'themselves or in their neighbour- 

 hood. 



The discussion of analyses given in this paper shows that 

 this mineral is probably in some cases at least of the con- 

 stitution indicated by the formula — 



3 Si0 3 + K 2 3 + 5 KO + 4 R 2 



— that is, of chlorite. But the mineral, as might have been 

 expected from its mode of formation, through the altera- 

 tion of mica and of amphibole — perhaps not always under 

 precisely the same conditions — does not seem in all cases to 

 be of the same structure or composition. The chloritic 

 minerals occurring in the quartz-mica-diorite under con- 

 sideration I found to be uniaxial, or to have an extremely 

 small optic-axial angle. I could not determine which it was, 

 as no section was precisely parallel to the basal cleavage. In 

 slices perpendicular to that direction it is dichoric in 



