303 



I 0> Lepidomelano from granite, Ballyellin, Co. Carlow, Leinster. (HATTGHTON). 

 II. Lepidomelane from granite, Ballygohen, Co. Donegal. (HAUGHTON). 



III. Lepidomelane, vein in granite, Tongue, Sutherland. (HEDDI/E). 



IV. Haughtonite from granite vein, Roueval, Harris. (HEDDLE). 

 V. Haughtonite from vein in gneiss, Kinnaird Head. (HEDDLE). 



VI. Haughtonite from granite, Ben Stack, Sutherland. (HEDDLB). 



The black micas are easily affected by surface agencies when they 

 become green and finally pass over into chlorite. The alteration often takes 

 place along cleavage planes so that green and brown varieties frequently 

 alternate with each other. In the volcanic rocks and in plutonic rocks 

 with cataclastic structures the plates are frequently bent and crumpled. 

 Inclusions of apatite, zircon and iron-ores are common ; those of zircon 

 are frequently surrounded by a deeply coloured zone in which the 

 pleochroism is more marked than in the rest of the section. 



The white micas belong for the most part to the muscovite group ; 

 lepidolite occurs in the granite of stanniferous districts but is more 

 common in veins than in the main mass of the rock. Typical muscovite 

 is present in certain granites but is absent from the corresponding volcanic 

 rocks. It occurs as thin plates the outlines of which are more irregular 

 than those of the black micas. The optic axial angle varies from 40 to 

 70. The acute bisectrix is negative as in all micas. 



Muscovite is colourless in thin sections. It possesses very strong 

 double refraction (7 a, 0.035) so that sections at right angles to the 

 principal cleavage give the pinks and greens of the higher orders even 

 in the ordinary rock sections. Analyses of muscovite invariably show a 

 certain amount of water. Those which contain about 5 or 6 p. c. are 

 sometimes termed margarodite. 



White micas are extensively developed in connection with certain 

 phases of dynamic metamorphism, especially when the deformation has 

 been accompanied by shearing along more or less definite planes. The 

 secondary mica may in these cases take the form of definite scales of 

 muscovite or may occur in extremely fine grained aggregates giving a 

 kind of crypto-crystalline reaction between crossed nicols. White micas 

 are also frequently developed apart from all mechanical actions. In all 

 cases they appear to be due to molecular changes in the felspar. Sericite 

 is a term applied to a mica which occurs in minute irregular scales and 

 scaly aggregates. It is identical with muscovite in composition. 



The white micas, whether original or secondary, are very stable under 

 the conditions which prevail at the surface of the earth and do not 

 appear to undergo chemical change in the processes of denudation and 



(1) I. and II. are quoted from Q.J.G.S., Vol. XV., p. 12;>; III., IV., V. and VI. from 

 Trans. Roy. Soc., Edin. Vol. XXIX., p. 1. 



