60 
SPOLIA ZEYLANICA. 
The form of the crystals, however, shows that the mineral is not 
tetragonal; it appears to be orthorhombic or monoclinic. The 
oriented sections which I am having prepared will probably clear 
up this point. In any case I do not at present regard the mineral 
as thorite, although its chemical composition is undoubtedly 
similar to that of thorite. 
As has already been pointed out, the occurrence of thorium¬ 
bearing minerals (one said to contain a higher percentage of thoria 
than any mineral previously known) in Ceylon is of great 
scientific interest, and if they are present in sufficient amount will 
be of considerable commercial-importance owing to the use of 
thoria for the manufacture of incandescent gas mantles. The 10 
or 12 per cent, of uranium oxide alone gives a value of £20 or £30 
a ton. The very small amount of radium present is of no practical 
importance. 
II.— Corundum, Sillimanite, &c. 
Remarkable corundum-sillimanite rocks were found at Hal- 
dummulla (Uva). They occur all down the slope of Haldummulla 
estate, but not in situ , being derived from some point in the 
inaccessible hill above ; the fallen blocks are found as far down 
as the western end of Kalupahane estate. The minerals sillima¬ 
nite, corundum, orthoclase-microperthite, garnet, rutile, and 
ilmenite enter into the composition of these rocks. The principal 
types met with are : sillimanite-corundum rock, sillimanite-garnet 
rock, and sillimanite rock ; orthoclase-microperthite is present in 
subordinate and varying amount; ilmenite and rutile are accessory 
and do not occur together, but mutually replace each other ; the 
corundum and garnet also do not occur together in the same rock. 
The corundum forms violet-coloured hexagonal crystals, usually 
less than J inch in diameter ; the crystals have often a tabular 
developement, the forms c (0001), a (1120), and r (1011) are 
characteristic ; rhombohedral cleavage is well developed. The 
sillimanite occurs in parallel and radiating groups and single 
individuals, the latter generally idiomorphic (prismatic, giving 
rectangular cross sections). The crystallization is much coarser 
than is usual for sillimanite ; the largest crystals may reach a 
length of 2 inches or more and diameter one-tenth inch. The 
sillimanite has in the rock a pale gray colour, but is colourless in 
thin flakes. A more detailed account of these rocks will be else¬ 
where given. 
Sillimanite has been met with in the garnetiferous leptynites 
in some abundance, over a large area between Bandarawela on the 
one hand and the Bambarabotuwa district (Sabaragamuwa) on the 
other; also in small amount in a garnetiferous rock found at 
