ON THE RUBIES OF BURMA AND ASSOCIATED MINERALS. 
195 
some varieties of the rock, are somewhat rare, but in others very abundant. The 
pleochroism is intense. 
c and i), greyish and reddish-brown, 
a, yellow to red-brown. 
Absorption £ > ft > a. This is so great parallel to C and fj that the mineral 
appears black when traversed by rays vibrating in these directions. 
While the mass of the rocks is made up of these three minerals others appear in 
smaller quantities. 
Zircon is abundant, enclosed in all the minerals of the rock. When found in the 
biotite, pleochroic halos are exhibited by the latter mineral. Apatite is somewhat 
rare, as are also magnetite and titanoferrite. The latter sometimes exhibits the 
transformation to spliene, and the former to limonite, and occasionally into hematite. 
Garnet (almandine) is often a very abundant accessory constituent of the rocks, 
and not infrequently is found undergoing change along the cracks which traverse it. 
In some cases the garnet is so abundant that it must be regarded as an essential 
constituent. 
CordAerite , sillimanite (intergrown with biotite), graphite, with primary spliene, 
also occur in these rocks. 
These gneissic rocks vary greatly in the coarseness of their grain. The most 
abundant type is a moderately coarse-grained gneiss which occasionally exhibits 
large “ eyes,” composed of quartz and felspar, and with the foliated structure fairly 
well-marked. In other cases, the rock, is seen to be made up of more or less rounded 
grains of quartz and felspar, with some biotite and abundant garnets ; and it may 
then be called a “mica granulite.” On the other hand, the biotite occasionally 
increases in amount, while the felspar diminishes, and, the foliated structure of the 
rock becoming more pronounced, it approximates to a true mica-schist. Hornblende 
appears to occur but rarely in this series of rocks. 
The greatest interest attaches however not to these gneisses themselves, but to 
the remarkable rocks which form subordinate members of the series, and are found 
interfoliated with them. Some of these are of more acid composition than the 
gneisses, and may be classed as pegmatites and aplites, passing into quartzites ; 
others are of more basic composition, and include scapolite gneiss with various forms 
of pyroxene-gneiss, passing into pyroxenites and amphibolites. With these latter 
occur lazurite-diopside rocks (lapis-lazuli) and various forms of impure limestones 
(cipolines and calciphyres). We shall proceed to consider, firstly, the acid rocks 
intercalated with the gneiss series of Burma, then the basic rocks, and finally the 
remarkable limestones which Mr. Barrington Brown has shown to be the parent 
rock, both of the red corundum (rubies) and the red spinel (rubicelle or balas ruby). 
2 C 2 
