the Corundum Stone, and its Varieties, &c. 295 
it, made by Mr. Chenevix, has proved the truth of my obser- 
vations. 
Its semi-transparency, in this state, is more considerable, and 
approaches very nearly to complete transparency. Its colour is 
generally a beautiful topaz yellow, which sometimes inclines 
slightly to green. I have hitherto met with it only in pieces of 
an indeterminate and irregular form, the size of which, though 
more or less considerable, never exceeded that of a small nut. 
Its fracture is generally irregular, and often partially con- 
choid. In some pieces, however, may be perceived small particles 
which seem to have a laminated texture, the direction of the 
laminae being such as to announce the primitive crystal of the 
thallite; but I have never been able to bring this substance to 
the shape of that crystal, by any artificial division or fracture 
of it. 
Hornblende. This substance is that which is most constantly, 
and most abundantly, contained in the matrix now treated of. 
There are indeed some pieces of the matrix, wherein the pro- 
portion of hornblende is as great as in some granite rocks of 
which it constitutes the principal component part; and those 
pieces have an appearance very similar to that of such rocks. 
It is generally of a deep black colour, and opaque ; but I have 
sometimes seen it in the form of small elongated crystals, of a 
fine green colour, and transparent. Its texture is very evidently 
laminated ; and it is seldom that any determinate form can be 
perceived in it ; sometimes, however, the rhomboidal tetraedral 
form of its prism may be distinguished. 
Quartz. In this matrix is also found quartz, in small detached 
fragments, of an indeterminate shape. This substance, however, 
is by no means common ; on the contrary, of the various 
Q q 2. 
