556 rROFESsoii Jameson's speculations 
XL VI. — Speculations in regard to the Formation 
of Opal, Wood-stone, and Diamond. 
By Proiessor J ameson. 
(Read ^^d February/ 1823.) 
I SHALL now offer a few observations on the natural history 
of the Diamond, with the view of shewing that its geognos- 
,tical distribution and modes of formation are probably more 
varied than has been generally believed. As opal and 
hornstone agree in many of their geognostical relations with 
carbon, the essential ingredient of diamond, we shall first 
trace the various modes of distribution of these minerals, 
and then those of the diamond, in order to shew that they 
have been formed in the same manner, and that all of 
them appear to be still forming on the surface of the earth, 
in the newest alluvial formations, and probably even in 
vegetables. 
1. Opal — Opal, which is a hydrate of silica, and emi- 
nently distinguished by the beauty of its range of external 
characters;^— occurs in small veins and imJjedded portions, 
