ON THE DIAMOND, OPAL, &e. 559 
present the principal characters of petrified wood. Indeed, 
I think it probable^ that some of the petrified woods in 
cabinets, are portions of trees that have been silicified by 
the Hving powers of the vegetable, and not trunks or 
branches which have been petrified or silicified by a mere 
mineral process. 
S. i^iamowd— Having now shewn that opal and horn- 
stone extend in this series of rock-formations, from the pri- 
mitive 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 diamond, as is well known, is carbon in 
a pure and highly crystallised state, — and although carbon is 
a very generally distributed substance, it has hitherto occur- 
red 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 oxide of carbon, but of a difi'erent nature from that in 
glance-coal,— and, from information lately obtained from 
India, even carbon, in its purest state, in the form of 
diamond, is said to occur imbedded in a conglomerated 
quartz, subordinate to clay-slate. 
Greywacke, and other rocks of the transition class, con- 
tain graphite and glance-coal, but hitherto have alforded 
no traces of the diamond. Graphite and glance-coal occur 
in considerable beds in formations of the secondary class. 
The diamond, according to different authors, is met with 
in trap-tuffas, in sandstone, and in amygdaloids of the se- 
condary series. But the geognostical distribution of this 
gem does not appear to terminate here, for we are as- 
sured by those who have attended to its situation in the 
o o9. 
