ed 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.

[9] Van Hise, C. R., and Leith, C. K., Geology of the Lake Superior
region. _Mon. 52, U. S. Geol. Survey_, 1911, pp. 506-518; and references
there given.

CHAPTER IV

MINERAL RESOURCES--SOME GENERAL QUANTITATIVE CONSIDERATIONS

Of the 1,500 known mineral species, perhaps 200 figure in commerce as
m