qf Opal, Woodr-stone and Diamond . 165 
fctone. This remarkable specimen, which is 18 inches long, 5 
inches thick, and 8 inches broad, was torn from the interior of 
a log of teakwood ( Tectona grandis ), in one of the dock-yards 
at Calcutta. The carpenters, on sawing the log of teakwood, 
were arrested in their progress by a hard body, which they 
found to be interlaced with the fibres of the wood, and, on cut- 
ting round, extracted the specimen now on the table. This 
fact naturally led me to conjecture, that the mass of wood-stone 
had been secreted by the tree, and that, in this particular case, a 
greater quantity of silica than usual had been deposited ; in 
short, that this portion of the trunk of the tree had become si- 
licified, thus offering to our observation in vegetables, a case ana- 
logous to the ossifications that take place in the animal system. 
I was further led to suppose, that this wood might contain sili- 
ca in considerable quantity, as one of its constituent parts, a con- 
jecture which was confirmed by some experiments made by Dr 
Wollaston. Other woods appear also to contain silica, and these, 
in all probability, will occasionally have portions of their struc- 
ture highly impregnated with silica, forming masses which will 
present the principal characters of petrified wood. Indeed, 1 
think it probable, that some of the petrified woods in cabinets 
are portions of trees that have been silicified by the living powers 
of the vegetable, and not trunks or branches which have been 
petrified or silicified by a mere mineral process. 
3. Diamond . — Having now shewn that opal and hornstone 
extend in the series of rock-formations, from the primitive to 
the newest alluvial rocks, and that both appear to be forming in 
vegetables of particular kinds, we shall next endeavour to shew 
that the same is probably the case with the diamond. The dia^ 
mond, as is well known, is carbon in a pure and highly crystal- 
lised state, — and although carbon is a very generally distributed 
substance, it has hitherto occurred but very sparingly in its pure 
and crystallised state, or in that of the diamond. Primitive 
rocks, of almost every description, contain carbon, — either in the 
state of an acid, forming carbonic acid, as in the carbonates of 
lime and magnesia, — or in the state of an oxide, as in glance or 
metallic coal, — or in graphite or black lead, which is also an ox- 
ide of carbon, but of a different nature from that in glance-coal, 
—and, from information lately obtained from India, even carbon, 
