166 Professor Jameson Y Speculations on the Formation 
in its purest state, in the form of diamond, is said to occur im- 
bedded in a conglomerated qtiartz, subordinate to clay-slate. 
Grey wacke, and other rocks of the transition class, contain 
graphite and glance-coal, but hitherto have afforded no traces of 
the diamond. Graphite and glance-coal occur in considerable 
beds in formations of the secondary class. The diamond, ac- 
cording to different authors, is met with in trap-tuffas, in sand- 
stone, and in amygdaloids of the secondary series. But the 
geognostical distribution of this gem does not appear to terminate 
here, for we are assured by those who have attended to its situa- 
tion in the earth, that it is found in alluvial beds of clay, not 
as a secondary deposite, but as an original one; in short, that the 
diamond continues to form, or to use a more common language, 
to grow as in some alluvial districts in India. This opinion is 
not improbable, and nothing more seems to be necessary for the 
formation of the diamond in such situations, than time, or other 
favourable circumstances, for allowing portions of the carbona- 
ceous matter in the soil to be reduced to the adamantine state, 
and afterwards to coalesce, according to the laws of affinity, 
into the granular and crystallised form, — in short, 1 6 form dia- 
mond. The gradual formation of calcareous grains, crystals 
and masses of calcareous spar in clays, of siliceous compounds in 
similar rocks, appears to be occasioned by the gradual concen- 
tration of the calcareous and siliceous particles by some attrac- 
tive power, in the same manner as; we conceive diamonds may 
have been formed by the concentration of particles of carbon. 
The preceding details, in regard to opal and hornstone, na- 
turally lead us to inquire, if it is probable that the diamond, 
like these substances, is occasionally formed by the powers of 
vegetation ? Reasoning a priori , we would say it is much more 
likely that some plants would produce diamonds, than that they 
would secrete siliceous matter in a state fit to form opal and 
hornstone, because diamond is but carbon, the principal consti- 
tuent part of plants, in a peculiar state ; whereas the silica of 
the opal and hornstone are subordinate ingredients in vegeta- 
tion. But a direct appeal to the characters of some woods 
seem to countenance the idea I some years ago suggested in the 
Society, that vegetables may contain carbonaceous matter ap- 
proaching to the adamantine state. Certain woods which have 
