24 
ECONOMIC GEOLOGY 
superseded, and the ores are attributed to water given off 
from the interior of the earth. This water is generally 
described as magmatic, but that term is ambiguous and is 
used for ore deposits with three different meanings. Harker 
(Vat. Hist. Igneous Rocks, 1909, p. 31) defines a magma as 
“molten rock material.” “The original molten rock 
matter is conveniently termed magma,” says Tyrrell (Prin- 
ciples Petrol., 1926, p. 46); Lindgren (Tr. Amer. I.M.E., 
Ixxiv, 1927, p. 77) gives a definition in the language of 
physical chemistry which means the same, as he remarks 
{1bid., p. 76), magma is *‘ the material from which all our 
igneous rocks have consolidated.” In accordance with this 
petrographic use of the term many ores are described as 
magmatic, being attributed to the direct consolidation of 
molten rock material. R. H. Rastall (Geol. Met. Dep., 1923, 
P. 109) entitles one group of ores “ magmatic segregations ; 
ores actually separated from igneous magmas by crystal 
lization ; ” and W. Lindgren devotes a chapter (Min Dep., 
1013, pp. 735-72) to ‘‘ Mineral Deposits formed by Concen- 
tration in Molten Magmas.” The magnetites of Taberg, of 
Lapland, and the Adirondacks (cf. p. 140), the nickel ores of 
Sudbury and South Africa (cf. pp. 114-19), and the tin of South 
Africa (Recknagel. T7. G. Soc. S. Afr., xii, 1910, p. 128), have 
been represented as magmatic in this sense of the term ; 
but in the account of the ores in the following chapters it is 
claimed that the ores thus formed are few and of little present 
commercial importance. 
The second set of ores called magmatic are due to water 
of magmatic origin. This use of the term was begun by 
J.H.L. Vogt (Z. prakt. G., 1804, p. 381), who extended it to 
emanations that are given off after the consolidation of the 
molten material and are discharged through hot springs, 
fumaroles, and solfataras. Many authors have followed 
Vogt’s lead and accepted as magmatic all ores due to water 
of magmatic origin; thus W. H. Goodchild (Ming. Mag. 
1918, xviii and xix) includes even the Banket of the Rand, 
and truly says ‘they are ores of endless variety " (ibid. 
xviii, p. 135). According to this usage many beds of tufa, 
L Cf. Gregory, ©“ Magmatic Ores,” 7¥, Faraday Soc., xx, 1925, pp. 
149-38.