This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Limestones are mainly derived from the Devonian, Carboniferous, Permian, and Oolitic formations. They consist, as a rule, almost entirely of carbonate of lime, or of admixture of lime and magnesia. Some, however, contain a considerable proportion of magnesia, while there are others with varying percentages of silicates, and a little alumina and iron. Some have fossil remains of sufficient size to prevent their use for decorative work, but these are generally massive and durable, and thus useful for positions where strength is the principal requirement.
The limestones found in this formation are often crystalline, being metamorphosed rocks and true marbles, and as such are susceptible of a high degree of polish. The quarries of this class are mainly found in Devonshire, near Plymouth, Babbacombe, and Ipplepen, and also to the north of the county, as at Staverton.
In colour these marbles vary much, - black, grey russet, etc., being found, some varieties being veined with streaks of white, yellow, or other tints.
In general the limestones of this formation are hard and somewhat crystalline, in many instances containing the remains of sea-lilies, corals, and nautilus shells. Trilobites are rarely found. Marbles are found in various parts of the Midlands (chiefly Derbyshire) belonging to this formation. Among the more important quarries are the following: -
A limestone composed of an intimate admixture of carbonate of lime and clayey matter. It is of a dull white with a green tinge. Is a fair weathering stone if carefully chosen. Its weight is only 117 lbs. per cubic foot.
A fine crystalline limestone capable of taking a good polish. It is a nearly pure carbonate of lime, with shelly fragments and fossils of encrinites embedded within the mass. Colour, light grey. It is of good weathering quality. Constituents - carbonate of lime, 98.5 per cent.; iron and alumina, a trace; silica, 0.75 per cent. Weight, 160 lbs. per cubic foot. Crushing resistance, 455 tons per square foot.
A very crystalline and compact stone, capable of a good polish. Colour, light cream. Is a good sound weathering stone. Nearly pure carbonate of lime. Weight, 136 lbs. per cubic foot.
A crystalline limestone of a close-grained texture, taking a high polish. There are two distinct beds of this stone, differing to some extent in their characteristics. They are very compact and durable, and of good weathering properties. Weight, 168 lbs. per cubic foot.
To the Permian belong the Magnesian limestones or Dolomites. These stones are generally chemical precipitates, naturally deposited in ancient salt lakes. Their general characteristics are ease of working and compactness, the granulation being very even and small. One peculiarity of this stone is the presence of silica in varying proportions. The best stone is, however, that which attains the nearest equality between the carbonate of lime and magnesia.
The more compact and homogeneous in structure the more durable is the stone likely to be.
The best known quarries are here enumerated.
A good ordinary building stone, not very crystalline and of a pleasing light brown colour. Can be procured in good sized blocks. Weight, about 132 lbs. per cubic foot. Crushing resistance, 155 tons per square foot.
Is generally known under the term " Dolomite," owing to its composition. This stone is of good quality, and useful for general building purposes. Can be obtained in large blocks of good scantling. The constituents are as follows - silica, 52 per cent.; carbonate of magnesia, 18 per cent.; carbonate of lime, 27 per cent.; iron etc., about 1 per cent. Weight, 150 lbs. per cubic foot.
Generally similar in structure to the above, the colour being a pleasing brown with a rosy tinge. Its constituents vary somewhat from the white as follows: - silica, 50 per cent.; carbonate of magnesia, 16 per cent.; carbonate of lime, 27 per cent.; iron, and clay, 3 per cent. Weight, 148 lbs. per cubic foot.
A highly crystalline stone of compact and even texture. A good weathering stone. Can only be had in moderate sized blocks. This stone is more of the nature of the "true" dolomite than the Mansfields. In colour it is yellow. Its constituents are - carbonate of magnesia, 40 per cent.; carbonate of lime, 51 per cent.; silica, 3.5 per cent.; iron, etc., 2 per cent. Weight per cubic foot, about 152 lbs.
Somewhat crystalline although of a granular texture. This is a good building stone. It is, however, most useful for steps, landings, engine beds, and all heavy work. It can be procured in fairly large blocks. Colour, cream. Weight, 143 lbs. per cubic foot.
A very useful stone of a light brown colour. Weight, 134 lbs. per cubic foot.
A somewhat friable stone, containing the carbonates of magnesia and lime. To be obtained in good sized blocks up to 4 feet thick. Colour, cream. Weight, 127 lbs. per cubic foot.
A slightly crystalline stone, useful for general building purposes. It is similar in its constituents to Cadeby. Colour, a light cream. Can be procured in large blocks. Weight, 138 lbs. per cubic foot. The proportions are - carbonate of magnesia, 41 per cent.; carbonate of lime, 54 per cent.; silica, 3 per cent.; iron, a small quantity.
A good building stone of a light cream colour. Like many limestones, unless carefully selected it is apt to be attacked and to deteriorate under the influence of a smoky atmosphere or where much rain is prevalent. Is composed almost entirely of carbonates of magnesia and lime in the proportions of 42 per cent. of the former and 56 per cent. of the latter. Weight, 137 lbs. per cubic foot.