Many limestones contain carbonate of magnesia, but those with less than 15 per cent do not come into the class now under consideration.
The better varieties of magnesian limestone are those in which there is at least 40 per cent of carbonate of magnesia, with 4 or 5 per cent of silica.
When the magnesia is present in the proportion of one molecule of carbonate of magnesia to one molecule of carbonate of lime (i.e. 54.18 carb. magnesia and 45.82 carb. lime), the stone is called a Dolomite.1
Professor Daniel states that the nearer a magnesian limestone approaches dolomite in composition, the more durable it is likely to be.
Mr. C. Smith says, "In the formation of dolomite, some peculiar combination takes place between the molecules of each substance; they possess some inherent power, by which the invisible or minutest particles intermix and unite with each other so intimately as to be inseparable by mechanical means. On examining with a highly magnifying power a specimen of genuine magnesian limestone, such as that of Bolsover Moor, it will be found not composed of two sorts of crystals, some formed of carbonate of lime others of carbonate of magnesia, but the entire mass of stone is made up of rhomboids each of which contains both the earths homogeneously crystallised together. When this is the case, we know by practical observation that the stone is extremely durable."2
Some magnesian limestones contain sand, in which case their weathering qualities are greatly injured.
Some are peculiarly subject to the attacks of sulphuric acid, which forms a soluble sulphate of magnesia easily washed away.
The following Table gives analyses of some of the principal magnesian limestones. The red and white Mansfield contain a large proportion of silica and are generally classed among the sandstones (see p. 38).
1 After a French geologist Dolomieu, who was the first discoverer of this mineral in the Alps. 2 Smith's Lithology.
6 Mansfield, Red.
8 Mansfield Wood-house.
Carbonate of Magnesia
Carbonate of Lime
Iron and Alumina
Water and Loss
Cols. 2 3 4 5 from the Report of the Royal Commission. 16 7 8 Smith's Lithology.
9 Page's Economic Geology. 10 11 Builder, 20th November 1886.
A few of the most noted varieties of limestone used in this country will now be described, after which, at p. 67, will be given a list of some of the principal quarries in Great Britain and Ireland.
Bath Stone is one of the best known and most extensively used building stones in this country.
This stone is obtained from that division of the Oolitic formation which is known as the Great or Bath Oolitic group. Geologically speaking it lies below the Portland stone, being separated from it by the Kimmeridge clay, coral rag, and Oxford clay.
Most of it is of a fine even grain, composed chiefly of carbonate of lime - sometimes interspersed with shelly fragments.
Some of the varieties of this stone contain sand-cracks, vents, clay-balls, etc.; these should of course be avoided.
The quarries are worked by tunnelling, and the stone is produced in blocks up to 5 or 6 feet deep, and weighing as much as 10 or 12 tons.
It is important that Bath stone should be quarried in summer when it is freed from the ground moisture or quarry sap. If quarried in winter it is very likely to fall to pieces with the first frost.
The stone is very soft when first quarried, but hardens upon exposure to the air (see p. 49). It is important that it should be placed on, or parallel to its natural bed (see p. 9).
As it is obtainable in large blocks, and is easily worked, it is particularly valuable for mouldings and carved work.