Speculations on the Formation of Opal c J6& 
in a primitive track, into which it is seen to pass* Before the 
blowpipe it changes, although with difficulty, into a blackish 
globule, which has sometimes externally a metallic lustre-; with a 
little natron it changes into a blackish green globule ; with more 
natron into a rough slaggy mass ; and with borax into, a green 
glass. Is partly soluble in muriatic acid. 
I found its chemical composition to be^as follows : 
Silica, 
49.40 
Magnesia, 
- 
10.67 
Alumine, 
13.80 
Phosphoric Acid, 
2.68 
Peroxide of Iron, 
18.17 
Water, 
r 
4.38 
99.10 
It seems therefore to be a mixture of 
MS 2 + 2fe S 2 + 3 AS 2 with MqF -f Zaq. 
Art. XVII. — Speculations, in regard to the Formation of Opal ^ 
Wood-stone , and Diamond. By Professor, Jameson 
I shall now offer a few observations on the natural history of 
the Diamond, with the view of shewing that its geognostical dis- 
tribution 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, m 
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 eminently 
distinguished by the beauty of its range of external characters,— 
occurs in small veins and imbedded portions, in various primi- 
tive rocks. But its principal distribution is in rocks of the se- 
condary class, particularly in traps and porphyries. In these it 
is arranged in veins, drusy cavities, and imbedded masses, and 
assumes the various forms of precious opal, common opal, semi- 
From part 2. of Vol. IV. of the Memoirs of the Wernerian Natural History 
Society, just published. 
L 9 
