ON THE RUBIES OE BURMA AND ASSOCIATED MINERALS. 
199 
On a ridge near this same mountain there occurs a very similar rock, containing 
orthoclase and some plagioclase felspar, the brown hornblende, and a little biotite. 
The great point of interest about this rock, however, is the circumstance that both 
scapolite and a large amount of caleite have been developed in it, apparently at the 
expense of a lime-felspar (see fig. 17, p. 203). 
Similar rocks, intermediate in character between the acid rocks and the normal 
gneisses, are found as fragments in the ruby-gravels. Thus, in the Hmyaudwin 
No. 15a, at Mogok, a specimen of a pegmatite or very coarse gneiss was found which 
contains orthoclase crystals, sometimes including patches of plagioclase, and tufts of 
fibrolite needles ; plagioclase felspar sometimes passing into scapolite ; an abundance 
of a dark brown biotite, which is sensibly biaxial but with a small angle between the 
optic axes ; and a few grains of a pale-green augite (sahlite) exhibiting twin 
lamellae. 
3. Basic Rocks. — Pyroxene- and Seapolite-Gneisses and Granulites, Pyroxenites, <xc. 
It is in the more basic rocks associated with the gneissic series in Burma, with 
their closely associated crystalline limestones, that the chief interest of the geologist 
is centred ; for it is undoubtedly in connexion with these rocks that the rubies, 
spinels, and other gem-minerals are found. These rocks contain, in addition to a 
lime-felspar (anorthite or labradorite), several varieties of pyroxene (white and green 
diopside, sahlite, and hypersthene), which may be replaced by hornblende or biotite, 
and in many cases a considerable quantity of scapolite, wollastonite, and calcite. 
Bucks of this class occur in bands which are subordinate to the gneiss of the 
district, and are common both in the Mogok valley and in the Injouk valley. 
Thicker masses of the same rock are found intercalated in the gneisses on the ridge 
between Mogok and Bernardmyo. The frequency of fragments of these rocks in the 
ruby-bearing gravels testifies to their very wide distribution. 
The rocks present a characteristic dark greenish-grey colour, often accompanied by 
a greasy lustre, and are thus easily distinguished from the ordinary gneisses with 
which they are associated. 
The coarser grained rocks of this class may be distinguished as pyroxene-gneisses, 
or as pyroxene-scapolite-gneisses, when the latter mineral is largely developed in 
them. These rocks appear to the eye as coarse aggregates of a grey felspar (anorthite, 
or a variety like bytownite) and a black or very dark-green augite, and various 
accessory minerals, among which sphene is the most abundant and conspicuous. 
Very coarse varieties of these pyroxene gneisses occur as fragments in the 
Hmyaudwins No. 3, at Mogok, and No. 27, in the Kyatpyen districts ; the rocks have 
suffered somewhat from atmospheric disintegration, but their essential characteristics 
can be made out in thin sections under the microscope. 
The felspars exhibit lamellar twinning in a very local manner, the lamellae dying 
out in certain portions of the crystals, and large areas of the latter being altogether 
