380 
Proceedings of the Royal Irish Academy. 
darker occur nearer the centre ; usually green prevails in the middle of 
a crystal, while the outer half or so may be colourless. The extreme 
margin generally consists of a thin layer of a fibrous undetermined 
mineral, the fibres standing at right angles to the direction of the edge. 
The difference in colour is associated with difference in specific 
gravity, which ranges from 2' 8 to 3*2 or possibly over. The lighter 
varieties are also the lighter in colour, and contain most lithium. 
Yery sharply defined " schlag-figuren " were obtained, and the 
plane of the optic axes was found to coincide in direction with the 
leading ray, i.e. with the clino-diagonal section or plane of symmetry. 
Hence the mica must be referred to the species Zinnwaldite, which has 
not hitherto been recorded as occurring in Ireland, the only previously 
known lithium-bearing mica in this country being lepidolite, which has 
been found in County Tyrone. 
The angle of the optic axes differs in different individuals and in 
different parts of the same crystal, varying apparently with the colour 
and density : thus the heavier dark-green centre of one crystal gave 
an angle of 44° 4'. On passing from the centre to the margin gradually 
increasing angles were measured, till at a place as near the edge as 
could be examined an angle of 52° 6' was observed. In my paper on 
the Leinster granites I have described a similar difference as dis- 
tinguishing the more central and more marginal parts of the muscovite 
crystals from Three Eock Mountain, and have explained it as the result 
of a passage from a more ferro-magnesian to a more alumino-alkaline 
mica occurring during the growth of the crystal, and presenting a 
very suggestive resemblance to the change in composition which 
accompanies the growth of many zonal felspars. In the present case 
a similar explanation applies ; observations on the specific gravity, 
colour, zonal structure, and change in the value of the angle of the 
optic axes combine to prove that in the zinnwaldite crystal a passage 
can generally be traced from a more ferro-magnesian mica (meroxene 
in this case) to a more alumino-alkaline mica, on passing from the 
centre of the crystal outwards. 
The change may result from changes in the composition of the 
successive layers added in the way of growth, or it may be partly due 
to successive encroachments of new material from without; in the 
muscovite from Three Rock Mountain there is reason to believe that 
the latter process has played an important part. 
