lowly spread through the New England states; but for
many years the home-made article was inferior to that imported from Europe.

The Dutch and the Germans were the great brickmakers of Europe during the
middle ages, although the Italians, from the 14th to the 15th century,
revived and developed the art of decorative brick-work or terra-cotta, and
discovered the method of applying coloured enamels to these materials.
Under the Della Robbias, in the 15th century, some of the finest work of
this class that the world has seen was executed, but it can scarcely be
included under brickwork.

_Brick Clays_.--All clays are the result of the denudation and
decomposition of felspathic and siliceous rocks, and consist of the fine
insoluble particles which have been carried in suspension in water and
deposited in geologic basins according to their specific gravity and degree
of fineness (see CLAY). These deposits have been formed in all geologic
epochs from the "Recent" to the "Cambrian," and they vary in hardness from
the soft and plastic "alluvial" clays to the hard and rock-like shales and
slates of the older formations. The alluvial and drift clays (which were
alone used for brickmaking until modern times) are found near the surface,
are readily worked and require little preparation, whereas the older
sedimentary deposits are often difficult to work and necessitate the use of
heavy machinery. These older shales, or rocky clays, may be brought into
plastic condition by long weathering (_i.e._ by exposure to rain, frost and
sun) or by crushing and grinding in water, and they then resemble ordinary
alluvial clays in every respect.

The clays or earths from which burnt bricks are made may be divided into
two principal types, according to chemical composition: (1) Clays or shales
containing only a small percentage of carbonate of lime and consisting
chiefly of hydrated aluminium silicates (the "true clay substance") with
more or less sand, undecomposed grains of felspar, and oxide or carbonate
of iron; these clays usually burn to a buff, salmon or red colour; (2)
Clays containing a considerable percentage of carbonate of lime in addition
to the substances above mentioned. These latter clay deposits are known as
"marls,"[1] and may contain as much as 40% of chalk. They burn to a
sulphur-yellow colour which is quite distinctive.

Brick clays of class (1) are very widely distributed, and have a more
extensive geological range than th