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cooling, They do not contain fossils or stratification but are sometimes made more
or less of mixtures of crystalline materials, fine or coarse. The most important
component minerals are feldspar, quartz, pyroxene, and hornblende. Granite is
most representative of the class. Other igneous rocks are diorite, basalt, etc.
Metamorphic rocks are sedimentary or igneous rocks changed in character by
mechanical movement of the earth's crust, by the chemical action of liquids and
gases, or by heat. They are highly crystalline, with a parallel structure at times
closely resembling stratification. The principal types are gneiss, mica-schist,
quartzite, slate, and marble. Gneisses are rocks containing feldspar, quartz, and
mica, with parallel or laminated structure. Mica-schist is a friable, slaty rock
containing quartz and mica, often dotted with common red garnets. Quartzite is
sandstone so firmly cemented by silica that fractures take place through the
quartz grains instead of around them. Slate rock originates from fine-grained,
sedimentary rocks; it is closely related to shale, and is sometimes called argillite
with reference to its origin from clay. Marble is the name given to the meta-
morphic condition of sedimentary rocks formed by lime deposits such as lime-
stone and chalk. Generally, marks of bedding, fossils, etc., are effaced and the
material is converted into crystalline grains of calcite. Dolomite marble is com-
posed of a double carbonate of lime and magnesia in crystalline and granular
masses. Pure marble is white. The figures and coloring of ornamental varieties
are caused by impurities; the reds and yellows result from oxide of iron and the
neutral tones from organic matter. Marble is hard, compact, and takes a good
polish.
The use of stone as a paint support is largely architectural. That much of
stone, both flat and sculptured, carried paint in classical times is a commonplace
of history and it is to be supposed that much of the technical writing about how
to paint on stone had this end in view. By the XVI century in Europe, however,
it is found to carry occasional independent compositions. Perhaps the painter
most devoted to it was Alessandro Turchi, the Italian (1582-*: 1648), but northern
painters used it also.
Stretcher. This is a frame, usually rectangular in shape and made of wood,
over which a fabric is pulled and held taut for purposes of painting. Thompson
(Tempera Painting, pp. 15-17) describes the use of stretchers in preparing
canvas. (See also figure 6, p. 286, and p. 312.)
Stucco is a term that has been loosely and broadly applied to a variety of
materials used in the plastic state for the covering of walls, for the preparation
of decorative details on buildings, and for the making of figures and reliefs. The
term is often used synonymously with lime plaster. It has been given to gypsum
and to pozzolanic materials, particularly when these are in figures and reliefs.
It is sometimes given to plaster of Paris that has been hardened by something
like lime water. Modern, technical use generally confines it to a plaster for ex-
terior walls or other external surfaces of any building or structure. Here it is used