Compact limestone consists of lime carbonate 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. 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 setts and road metal under a light traffic. They weigh 153 to 172 lb. per cub. ft., and absorb very little water, taking up generally less than 1 per cent. by -weight in 24 hours.