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.
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.
These limestones consist of grains of carbonate of lime cemented together by the same substance, or by some mixture of carbonate of lime with silica or alumina.
They are generally found in the Oolitic (or eggstone) formation. The grains vary greatly in size. In some cases they are very small and uniform, very few being of a larger size, as in Caen stone. When the whole of the grains are somewhat larger, as in Ketton stone, they constitute what are called "Roestones" the structure resembling that of the roe of a fish. When the grains are still larger, as big as peas, the stones are known as Pisolites, or pea stones.
These stones nearly all contain fossil shells. In some cases the shell)" matter occurs in larger quantity than the grains. They are then called shelly granular limestones.
The colour of these stones is very variable, being sometimes white, light yellow, light brown, or cream-coloured.
The granular limestones are generally soft and somewhat absorbent. They are therefore liable to the attacks of acid atmospheres, and of frost, but otherwise are fairly durable.
This stone "is generally obtainable in large blocks, and it is often difficult when the stone has been sawn to detect its natural bed. This may be sometimes done by directing a jet of water on the side of the block, and it is well to do this as it is of great importance with some of the less durable sorts that they should be set upon their natural bed." l
The weight of this class of stone varies from 116 to 151 lbs., the lighter and more absorbent stones being, as might be expected, less durable than those of a more compact order.
Their absorption of water in twenty-four hours is hardly ever less than 4 per cent of their weight, while it is sometimes as much as 12 per cent.1
This class affords some of the principal building stones of this country, many of which will hereafter be described more in detail.
The very fine grained stones may be represented by Chilmark (see page 63); those with larger grains by Portland, Ancaster, and Pains wick; and those with large spherical grains by Ketton and Casterton; while Bath stone has large egg-shaped grains.
Some of these stones - such, for example, as certain varieties of Portland - are well adapted for outdoor work; others - such as Bath, Caen, Painswick - for internal work, carving, etc.; while some of the harder kinds - such as Seacombe, Painswick, and some of the beds of Chilmark and Portland - are adapted for internal staircases where there is not likely to be much wear.