The chemical composition of the different varieties is almost the same, and it may therefore be given at once for the whole.
The following is the analysis made by Professors Daniel and Wheatstone for the commissioners who selected the stone for the Houses of Parliament: -
Carbonate of lime...
Carbonate of magnesia ....
Iron and alumina
• • • •
Water and loss .
• • • •
It will be seen that the stone consists almost entirely of carbonate of lime. "The most durable stone has its cementing matter in a solid and half crystalline state; in the least durable stone it is in an earthy and powdery state." l
Clay and shingly matter; debris of Purbeck stone.
Slaty beds of stone.
Bacon tier, with layers of sand.
Dirt bed, containing fossil trees (Cycades).
Top cap, 8 or 10 feet thick. Scull cap.
Boach (true), 2 or 3 feet thick. Whitbed, 8 to 10 feet thick.
Curf and Basebed roacb. Basebed stone, 5 or 6 feet thick.
Quarried by means of wedges and levers (no blasting).
Flat beds or flinty tiers.
1Beport of Commissioners respecting stone to be used in building the new Houses of Parliament.
Roach, or True Roach as it is sometimes called, is a mass of fossils united by a cement composed of carbonate of lime.
The stone also contains a great number of cavities, large and small, being the moulds left by fossils that have dropped out.
Most of the fossils are merely casts, but in some cases portions of the shell are left.
The true roach may be distinguished from the other as it contains the peculiar fossil shown in Fig. 3, and known as the "Portland screw," which is never found in the bastard roach. This is an important distinction, as the true roach weathers far better than the bastard roach.
True roach is one of the best stones that can be used for heavy engineering works.
It is remarkably tough and strong, weathers admirably, and resists the action of water particularly well.
It has been much used ,for fortifications, breakwaters, dock and sea walls, and is suitable for massive plinths or other ashlar work where a rough face is appropriate; but the numerous cavities it contains render it unsuitable for fine work, and for positions where smooth faces or sharp clear arrises are required.
The colour of true roach is a very light brown. Whitbed. - This is the most valuable bed of the Portland series. It immediately underlies the true roach stone, which is firmly attached to its upper surface.
The stone consists of fine oolitic grains, well cemented together, and interspersed occasionally with a small amount of shelly matter. The cementing material is hard and crystalline.
Good whitbed stone resists the weather admirably. It is easily dressed to a smooth surface, and will take a very fine arris. It is suitable for the finest class of ashlar work. Some of it is too hard and not sufficiently uniform in texture for carving; other blocks are quite fit for the most intricate work. When examined through a microscope the grains of whitbed will be found to have a more oolitic or roe-like appearance than those of basebed (see p. 63). The grain is also more open, and the cementing material is stronger.
The colour is generally white, or nearly so, but some of the best stone has a decidedly brown tint.
It is unfortunate that there is no more marked distinction between whitbed and basebed, as the weathering qualities of the latter are greatly inferior. Base-bed is fit only for internal work, and great disappointment is caused when it is used, mistaking it for whitbed, in external work exposed to trying atmospheres. The carver, however, prefers basebed, though it is not so durable, because it looks better and is more easily worked.
Bastard Roach, Basebed Roach, or Curf.2 - This stone resembles true roach in appearance, being a mass of fossils and cavities. The cementing material is, however, inferior, the stone does not weather well, and it is not used for building or engineering works, except in the immediate locality.
The thickness of basebed roach varies considerably in different quarries. In some there is scarcely any, in others the bed has a thickness of from 12 to 24 inches, or even more.
Fig. 3.1. Cast of the shell known as the " Portland Screw," Cerithium Portlandicum.
1 From Lyell's Geology.
2 Sometimes written "Kerf."
These beds are sometimes interspersed with thin layers of flint.
Basebed is not easily distinguished from whitbed. The external appearance of the stone from both beds is almost exactly the same.
The basebed is, however, more uniform in structure, and freer from shelly matter. Its weathering qualities are not so good as those of whitbed, but as it is softer to work, it is often preferred by masons, and is known in the market as best-bed.
If basebed be required for external work, it should be seasoned for a year before use, in order that it may have every chance of weathering well.
The stone from this bed is well adapted for internal work and carving of the highest class.
The Portland stone quarries are worked from open faces. Blocks of great size, such as 10 or 12 tons (140 or 170 cubic feet), can easily be procured.
After experimenting upon stone from the different beds, Professor Abel reported that "on the whole the evidence may be considered a little in favour of the opinion that an improvement in the strength of the stone is effected, to some extent, by seasoning." l
The whole island of Portland is full of quarries, each of which produces the different beds of 6tone above described.
The commissioners reported that "the best stone is in the N.E. part of the island, the worst in the S.W. part."
Several of the quarries belong to the Government, but some of the best are in private hands, and the stone is worked in great quantities for the market.
The names of the principal quarries are Waycroft, Wide Street, Maggot, Weston Independent, Inmosthay, Tout, Westcliff, etc.
The Westcliff whitbed is considered the most durable, but it is hard to work; the whitbed quarried at the Bill is harder still.
Buildings in which used. - Portland stone, chiefly whitbed, was used for all buildings of importance erected in London from about 1600 to 1800. It was also used for the west front of St. Paul's, for the Horseguards, Somerset House, the General Post Olfice St. Martins-le-Grand, the India House and Foreign Offices in Downing Street, the Reform Club, and many other important buildings.