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Structural Materials

rocks, stones, layers, classification and unstratified

STRUCTURAL MATERIALS.

Classification of Natural Stones. The rocks from which the stones for building are selected are classified according to (1) their geological position, (2) their physical structure, and (3) their chemical composition.

Geological Classification. The geological position of rocks has but little connection with their properties as building materials. As a general rule, the more ancient rocks are the stronger and more durable; but to this there are many notable exceptions. According to the usual geological classification rocks are divided into three classes, viz.: Igneous, of which greenstone (trap), basalt, and lava are ex amples.

Metamorphic, comprising granite, slate, marble, etc.

Sedimentary, represented by sandstones, limestones, and clay.

Physical Classification. With respect to the structural char acter of their large masses, rocks are divided into two great classes: (1) the unstratified, (2) the stratified, according as they do or do not consist of flat layers.

The unstratified rocks are for the most part composed of an aggregation of crystalline grains firmly cemented together. Granite, trap, basalt, and lava are examples of this class. All the unstratified rocks are composed as it were of blocks which separate from each other when the rock decays or when struck violent blows. These natural joints are termed the line of cleavage or rift, and in all cutting or quarrying of unstratified rocks the work is much facilitated by taking advantage of them.

The stratified rocks consist of a series of parallel layers, evidently deposited from water, and originally horizontal, although in most cases they have become more or less inclined and curved by the action of disturbing forces. It is easier to divide them at the planes of divi

• sion between these layers than elsewhere. Besides its principal layers or strata, a mass of stratified rock is in general capable of division into thinner layers; and, although the surfaces of division of the thinner lay ers are often parallel to those of the strata, they are also often oblique or even perpendicular to them. This constitutes a laminated structure.

Laminated stones resist pressure more strongly in a direction perpendicular to their lamina than parallel to them; they are more tenacious in a direction parallel to their laminar than perpendicular to them; and they are more durable with the edges than with the sides of their laminw exposed to the weather. Therefore in building they should be placed with their laminar or "beds" perpendicular to the direction of greatest pressure, and with the edges of these laminar at the face of the wall.

Chemical Classification. The stones used in building are divided into three classes, each distinguished by the predominant mineral which forms the chief constituent, viz.: Silicious stones, of which granite, gneiss, and trap are examples. Argillaceous stones, of which clay, slate, and porphyry are examples.

Calcareous stones, represented by limestones aad marbles.