ON THE RUBIES OF BURMA AND ASSOCIATED MINERALS. 197 
this respect the Burma pegmatites appear to offer a striking contrast to those of 
Ceylon and Salem, in which microcline is the predominant felspar while plagioclase 
(oligoclase) is often present also. The last-named mineral is but seldom found in the 
Burma pegmatites. 
These orthoclases in the Burma pegmatites are not only remarkable for exhibiting 
the structure described by French petrographers as “ quartz of corrosion,” but for 
other peculiarities some of which will be noticed in the chapter on Mineralogy. The 
illustrations of the production of various kinds of schiller structure and of the 
alterations resulting in the play of colours characteristic of moonstone are of a very 
interesting character. 
All these structures in the orthoclase felspars are doubtless due to incipient 
alteration ; and in the fragments derived from these rocks, and included in the “ ruby- 
earth,” we find these changes carried out to their fullest extent, the orthoclase being 
sometimes converted into a kaolin, not unfrequently into a hydrous potash-mica 
(damourite or gilbertite) and at other times into an epidote. 
Epidote is often found developed along the cracks of these orthoclases, and in one 
instance (viz., in a specimen collected on the road between Mogok and Momeit), the 
epidote has the peculiar colour and pleochroism of withamite, due, no doubt, to the 
fact that it contains some manganese. In another case the epidote and felspar are 
found so curiously intergrown as to give rise to a structure, which, when seen upon 
the weathered surface of the rock, was thought to be of organic origin. 
The phenomena exhibited by these orthoclases in the various stages of their 
decomposition are of a very interesting kind and are worthy of the most careful 
study. In the ruby-earth, the orthoclase exhibiting the last stages of the alteration 
of the mineral is found, the masses crumbling between the fingers into a powder of 
kaolinite, muscovite, &c. 
The quartz in these pegmatites is of the same kind as that seen in the accom¬ 
panying gneisses, and is usually full of bands of inclusions ; perfectly clear quartz, 
probably of secondary origin, also occurs in them however. 
The mica is almost always a biaxial one—muscovite or damourite. Occasionally, 
however, biotite occurs, and these biotite rocks form a link between the pegmatites 
and the ordinary gneisses of the district. 
Among the accessory minerals, fibrolite (which is often enclosed in the orthoclase), 
garnet (almandine), zircon, and cordierite most frequently occur. Plagioclase felspar, 
as has been already pointed out, is comparatively rare in them. 
By the reduction in size of the grain of these pegmatites and a replacement of the 
graphic by the granulitic habit, the pegmatites pass into the aplites. These usually 
consist almost entirely of quartz and orthoclase in more or less rounded granules. 
Occasionally, as at Ingouk, near Bernardmyo, a rock of this class is found in which 
the quartz predominates over the orthoclase to such an extent that it passes in 
places into a granular quartzite. The only mineral which it contains, besides quartz, 
