Under all these conditions it appears that the maximum results are obtained at and near the surface. On the scale of the earth even the so-called deep veins may be regarded as deposits from solutions reaching the more open and cooler outer portions of the earth. However, valuable mineral deposits are found in the deepest rocks which have been exposed by erosion, and the question of what would be found at still greater depths, closer to the center of the earth, is a matter of pure speculation. Ultimately all minerals are derived from igneous sources within the earth. The direct contributions from these sources are only in small part of sufficient concentration to be of value; for the most part they need sorting and segregation under surface conditions. We can only speculate as to causes of the occurrence of valuable minerals in certain igneous rocks and not in others. Many granites are intruded into the outer shell of the earth, but only a few carry "minerals"; also, of a series of intrusions in the same locality, only one may carry valuable minerals. It is clear that in some fashion these minerals are primarily segregated within the earth. Causes of this segregation are so involved with the problem of the origin of the earth as a whole that no adequate explanation can yet be offered. Our inductive reasoning from known facts is as yet limited to the segregation within a given mass of magma, and even here the conditions are only dimly perceived. A discussion of these ultimate problems is beyond the scope of this book. FOOTNOTES: [4] Ransome, Frederick Leslie, Copper deposits near Superior, Arizona: _Bull. 540, U. S. Geol. Survey_, 1914, pp. 152-153; The copper deposits of Ray and Miami, Arizona: _Prof. Paper 115, U. S. Geol. Survey_, 1919, p. 156; Discussion: _Econ. Geol._, vol. 8, 1913, p. 721. [5] For more specific definitions of vertical zones of ore deposition in association with igneous rocks see Spurr, J. E., Theory of ore deposition: _Econ. Geol._, vol. 7, 1912, pp. 489-490; Lindgren, W., _Mineral deposits_, McGraw-Hill Book Co., 2d ed., 1919, Chapters XXIV-XXVI; and Emmons, W. H., _The principles of economic geology_, McGraw-Hill Book Co., 1918, Chapters VI-VIII. An excellent discussion of a case of vertical and areal zoning of minerals is contained in _Ore deposits of the Boulder batholith of Montana_, by Paul Billingsley and J. A. Grimes, Bull. Am. Inst. Min. Engrs., vol. 58, 1918, pp. 284-368. [6] Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore deposits of Utah: _Prof. Paper 111, U. S. Geol. Survey_, 1920, p. 201. [7] Leith, C. K., and Mead, W. J., _Metamorphic Geology_, Pt. 2, Henry Holt and Company, New York, 1915. [8] Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore deposits of Utah: _Prof. Paper 111, U. S. Geol. Survey_, 1920, pp. 152-158.