Previously formed igneous and sedimentary rocks under go changes because of metamorphic action of pressure and internal heat.

For example, because of metamorphic action granite becomes grasses, trap and basalt change to schist and laterite, limestone changes to marble, sandstone becomes quartzite, and mud-stone becomes slate.

Physical Classification.

Based on the structure, the rocks may be classified as:

• Stratified Rocks.

• Unstratified Rocks.

• Foliated Rocks.

Stratified Rocks.

Stratified Rocks.

These rocks have a layered structure. They possess planes of stratification or cleavage. They may be easily split along these planes. Sandstones, lime-stones, slate, etc. are examples of this class of stones.

Unstratified Rocks.

Unstratified Rocks.

These rocks aren’t stratified. They possess compact and comprehensible grains. They can’t be split into a thin slab. Granite, trap, marble, etc. are examples of this type of rock.

Foliated Rocks.

Foliated Rocks

These rocks have a tendency to split along a definite direction only. The direction is parallel to each other, as in the case of stratified rocks. This type of structure is very common in the case of metamorphic rocks.

Chemical Classification

On the basis of their chemical composition engineers prefer to classify rocks as:

• Silicious Rocks

• Argillaceous Rocks

• Calcareous Rocks

Silicious Rocks

Silicious Rocks

The main content of these rocks is silica. They are hard and durable. Examples of such rocks are granite, trap, sandstones, etc.

Argillaceous Rocks

Argillaceous Rocks

The main constituent of the rocks is argil, i.e., clay. These stones are tough and durable, but they are brittle. They can’t withstand shock. Slates and laterites are examples of this type of rock.

Calcareous Rocks

Calcareous Rocks

The main constituent of the rocks is calcium carbonate. Limestone is a calcareous rock of sedimentary origin, while marble is a calcareous rock of metamorphic origin.

Uses of Stones

Stones are used in the following civil engineering constructions:

(i) Stone masonry is used for the construction of walls, foundations, arches, and columns.

(ii) Stones are used for flooring.

(iii) Stone slabs are used as damp proof courses, lintels, as well as roofing materials.

(iv) Stones with good appearance are used for the face works of buildings. Polished marbles and granite are generally used for face works.

(v) Stones are used for paving of roads, footpaths and open spaces across the buildings.

(vi) Stones can also be used in the construction of piers and abutments of bridges, dams, and retaining walls.

(vii) Crushed stones together with graved are used to provide a base course for roads. When mixed with tar, they form finishing coat.

(viii) Crushed stones are used in the following works also:

(a) As a basic inert material in concrete

(b) For making artificial stones and building blocks

(c) As railway ballast

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