164 
ECONOMIC GEOLOGY 
diamonds are based on the belief that microscopic diamonds 
have been made artificially by the reduction of carbon at 
high temperatures and under great pressure in the presence 
of metallic iron, lithium, or basic silicates, or by the explosion 
of cordite in a bomb. These experiments led to the general 
conclusion that most diamonds were formed during the cooling 
of ultra-basic igneous rock and native iron. The artificial 
production of the diamond is however discredited by the 
work of Sir Charles Parsons in continuation of the researches 
described in his Bakerian Lecture (Phil. Tr. A., vol. 220, 
1926, pp. 67-107). He has examined the specimens ex- 
hibited to the British Association by Sir William Crookes, 
and analysed numerous specimens that he has himself 
made by the methods of Moissan and Crookes; he tells me 
that the crystals claimed as artificial diamonds are mostly 
silicon carbides, and other complex carbides of the impurities 
in the iron, viz. calcium, magnesium, chromium, etc., and 
that in his opinion no artificial diamond has yet been made.l 
He adds that for many years he believed the crystals, some 
of which he exhibited at the Royal Society in 1915, were 
diamonds, and that they burnt in oxygen at goo® C. ; but 
on repeating this test with rigorous care the most character- 
istic crystals were found uncorroded; some few had been 
whitened but retained their original form. 
Many facts, such as the nature of kimberlite, the matrix 
of the diamonds in the chief South African mines, and the 
presence in diamonds of hydrocarbons and apophyllite which 
would be decomposed at a high temperature (eg. J. C. 
Branner, Amer. Journ. Sci., (4), xxxiii, 1912, pp. 25-6, and 
G. F. Williams, T7. Amer. I.M E., xxxv, 1905, p. 451), indicate 
that the diamond was not formed at a high temperature. 
Its geological occurrence is in favour of its formation by the 
slow crystallization of carbon set free by the dissociation, prob- 
ably of a hydrocarbon or carbon tetrachloride at a moderate 
temperature, in material rendered viscous by superheated 
steam, as at the root of a mud volcano, or where a pheuma- 
tolytic solution is acting at an igneous contact or is pro- 
ducing pegmatite. 
tH. Le Chatelier remarks (Science et Industrie, 1923, P. 194), “ No 
one believes any more in the diamond of Moissan ”’