Analyses of igneous rocks indicate that nickel is present in the average igneous rock to the extent of 0.020[68] per cent. Of its occurrence, Clarke says:[69] “Very frequently detected in igneous rocks, probably as a constituent of olivine. * * * The presence of nickel is especially characteristic of magnesian igneous rocks, and it is generally associated in them with chromium.” Regarding copper he says: “Minute traces of this metal are often detected in igneous rocks, although they are rarely determined quantitatively.” It is estimated[70] that copper is present in the average igneous rock to the proportion of about 0.010 per cent., or only about half that of nickel, and that zinc and lead are present in even smaller proportion. Notwithstanding this greater abundance of nickel, the workable deposits of copper, lead, and zinc are much more widespread and production is correspondingly greater. “It can thus be said that nickel is less amenable to concentration by the agencies that tend to produce workable deposits than are the other metals mentioned (copper, lead and zinc).”[71] [68] F. W. CLARKE, “The Data of Geochemistry,” _Bull. 616_, U. S. Geological Survey, 1916, p. 27. [69] _Ibid._, p. 18. [70] Report of Royal Ontario Nickel Commission, 1917, p. 95. [71] _Ibid._ One would infer from the above that nickel deposits of consequence would most likely occur in connection with igneous rocks, especially those that are basic. This is indeed true; in fact, the only known nickel deposits of commercial or prospective commercial value occur in association with basic igneous rocks. The nickeliferous metallographic provinces of the world may be said to lie within petrographic provinces. Harker says:[72] “An examination of the rocks belonging to one great period of igneous activity, and of their actual distribution, enables us to distinguish areas of greater or less extent, within which the rocks present a less or greater degree of consanguinity, the law being that more marked specialization goes with narrower localization.” On the basis of this law and the fact noted by Clarke and mentioned above, nickeliferous metallographic provinces may be limited more closely than simply to areas of basic igneous rocks. They may be said to roughly coincide with areas of basic igneous rock characterized by minerals of the olivine group, these minerals having formed as a result of the high magnesium content of the rock magma. [72] HARKER, ALFRED, “The Natural History of Igneous Rocks,” 1909, p. 89.