friends. His popularity was as
great and as inexhaustible among his neighbours as among his
fellow-citizens generally. He pronounced upon himself a just judgment
when he wrote: "If any one desires to know the leading and paramount
object of my public life, the preservation of this Union will furnish
him the key."

See Calvin Colton, _The Works of Henry Clay_ (6 vols., New York, 1857;
new ed., 7 vols., New York, 1898), the first three volumes of which
are an account of Clay's "Life and Times"; Carl Schurz, _Henry Clay_
(2 vols., Boston, 1887), in the "American Statesmen" series; and the
life by T. Hart Clay (1910). (C. S.)

CLAY (from O. Eng. _claeg_, a word common in various forms to Teutonic
languages, cf. Ger. _Klei_), commonly defined as a fine-grained, almost
impalpable substance, very soft, more or less coherent when dry, plastic
and retentive of water when wet; it has an "earthy" odour when breathed
upon or moistened, and consists essentially of hydrous aluminium
silicate with various impurities. Of clay are formed a great number of
rocks, which collectively are known as "clay-rocks" or "pelitic rocks"
(from Gr. [Greek: pelos], clay), e.g. mudstone, shale, slate: these
exhibit in greater or less perfection the properties above described
according to their freedom from impurities. In nature, clays are rarely
free from foreign ingredients, many of which can be detected with the
unaided eye, while others may be observed by means of the microscope.
The commonest impurities are:--(1) organic matter, humus, &c.
(exemplified by clay-soils with an admixture of peat, oil shales,
carbonaceous shales); (2) fossils (such as plants in the shales of the
Lias and Coal Measures, shells in clays of all geological periods and in
fresh water marls); (3) carbonate of lime (rarely altogether absent, but
abundant in marls, cement-stones and argillaceous limestones); (4)
sulphide of iron, as pyrite or marcasite (when finely diffused, giving
the clay a dark grey-blue colour, which weathers to brown--e.g. London
Clay; also as nodules and concretions, e.g. Gault); (5) oxides of iron
(staining the clay bright red when ferric oxide, red ochre; yellow when
hydrous, e.g. yellow ochre); (6) sand or detrital silica (forming loams,
arenaceous clays, argillaceous sandstones, &c.). Less frequently present
are the following:--rock salt (Triassic clays, and marls of Cheshire,
&c.); gypsum (London Clay, Triassic clays); dolomite,