Composition and Structure. - Compact limestone consists of carbonate of lime either pure or in combination with sand or clay.

It is generally devoid of crystalline structure, of a dull earthy appearance, and of a dark blue, grey, black, or mottled colour.

In some cases, however, it is crystalline and full of organic remains. It is then properly known as a crystalline limestone.

Some of the Carboniferous limestones are of the compact class, also the Lias limestone, which contains a considerable amount of clay, and is used for making hydraulic lime; also Kentish Rag from the Cretaceous system, which is more fully described at page 64.

Uses

The compact limestones are good for building purposes, where their dull colour and the difficulty of working them are not objections.

They are useful for paving sets and road metal under a light traffic.

They are chiefly used however as flux in blast furnaces; for agriculture, bleaching, tanning, and other industrial purposes.

Weight

They weigh from 153 to 172 lbs. per cubic foot, and absorb very little water, taking up generally less than 1 per cent of its weight in twenty-four hours.