This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
These consist of grains of lime carbonate cemented together by the same substance, or by some mixture of lime carbonate with silica or alumina. They are generally found in the Oolitic formation. The grains vary greatly in size: in some cases they are very small and uniform, very few being of a larger size; when the whole of the grains are somewhat larger, they constitute what are called "Roestones," the structure resembling that of the roe of a fish; when the grains are as big as peas, the stones are known as " Pisolites," or pea stones. These stones nearly all contain fossil shells; in some cases, the shelly 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. They are generally soft and somewhat absorbent; therefore liable to the attacks of acid atmospheres, and of frost, but otherwise are fairly durable. The 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. The weight of this class of stone varies from 116 to 151 lb., the lighter and more absorbent stones being the less durable. Their absorption of water in 24 hours is hardly ever less than 4 per cent. of their weight, while it is sometimes as much as 12 per cent. This class affords some of the principal building stones of this country. The very fine grained stones may be represented by Chilmark; those with larger grains by Portland, Ancaster, and Painswick; and those with large spherical grains by Ketton and Castleton; while Bath stone has large egg-shaped grains. Some of these stones (e. g. 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 (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.