1 Hull's Building and Ornamental Stones.
2 Report on the Geology of Cornwall. Devon, and Somerset, by Sir H. de la Becha.
3 Hull. 4 wray.
In addition to the quartz grains there are often other substances, such as flakes of mica, fragments of limestone, argillaceous and carbonaceous matter, interspersed throughout the mass.
As the grains of quartz are imperishable, the weathering qualities of the stone depend upon the nature of the cementing substance, and on its powers of resistance under the atmosphere to which it is exposed.
Sometimes, however, the grains are of carbonate of lime, embedded in a siliceous cement; in this case the grains are the first to give way under the influence of the weather.
The colour is generally caused by the presence of iron.
Thus carbonate of iron 2 gives a bluish or greyish tint; anhydrous sesqui-oxide 3 a red colour; hydrated sesquioxides 4 gives various tints of brown or yellow, sometimes blue and green. In some cases the blue colour is produced by very finely disseminated iron pyrites, and in some by phosphate of iron.
The sandstones used for building are gener-ally classed as follows, either practically according to their physical characteristics, or scientifically according to their geological position or the nature of their constituents.
Liver Rock is the term applied, perhaps more in Scotland than in England, to the best and most homogeneous stone which comes out in large blocks, undivided by intersecting vertical and horizontal joints. In Yorkshire it is known as "Nell."
Flagstones are those which have a good natural cleavage, and split therefore easily into the thicknesses appropriate for paving of different kinds. The easy cleavage is generally caused by plates of mica in the beds.
Tilestones are flags from thin-bedded sandstones. They are split into layers - sometimes by standing them on their edges during frost, - and are much used in the North of England and in Scotland as a substitute for slates in covering roofs.
Freestone is a term applied to any stone that will work freely or easily with the mallet and chisel - such, for example, are the softer sandstones, and some of the limestones, including Bath, Caen, Portland, etc.
Grits are coarse-grained, strong, hard sandstones, deriving their name from the millstone grit formation in which they are found. These stones are very valuable for heavy engineering works, as they can be obtained in large blocks.
The geological formations from which the different varieties of sandstone are obtained are shown in the Tables, pp. 39-48, but any further notice of their classification from a geological point of view would be out of place in these Notes. With regard to their constituents, they may be divided into the following classes: -
1 Thus Mansfield stone is pale salmon colour; Red Corsehill, a brick red; Robin Hood, pale blue; Pennant, dark blue.
2 Ferrous Carbonats. 3 Ferric Oxide. 4 Ferric Hydrates.
Micaceous Sandstones are those which contain a very large proportion of mica, distributed over the planes of bedding.
Calcareous Sandstones contain a large proportion of carbonate of lime.
Felspathic Sandstones contain a large proportion of felspar, generally produced by the disintegration of granite or other felspathic rocks. The weathering qualities of these depend upon the nature of the felspar. (See p. 13.)
Metamorphic Sandstones are those which have been subjected to heat. They are too hard to work for building purposes, but are very suitable for breaking into road metal.
The recent fracture of a good sandstone, when examined through a powerful magnifying glass, should be bright, clean, and sharp, the grains well cemented together, and tolerably uniform in size. A dull and earthy appearance is the sign of a stone likely to decay.
A sandstone may be subjected to Smith's test or to Brard's test, described at page 11.
Recent experiments "led to the conclusion that any sandstone weighing less than 130 lbs. per cubic foot, absorbing more than 5 per cent of its weight of water in 24 hours, and effervescing anything but feebly with acid, is likely to be a second-class stone, as regards durability, where there is frost or much acid in the air; and it may be also said that a first-class sandstone should hardly do more than cloud the water with Mr. Smith's test."1
It is generally considered that the coarse-grained sandstones, such as the millstone grits, are the strongest and most durable. This, however, seems doubtful; at any rate, some of the finer-grained varieties are quite strong enough for any purpose, and seem to weather better than the others.
"It appears probable that for external purposes the finer-grained sandstones, laid on their natural bed, are better than those of coarser grain."l
In selecting sandstone for undercut work or for carving, care must be taken that the layers are thick; and it is of course important that stones should rest in most cases (see p. 37, Part I.) on their natural beds.
"Some of the harder sandstones are used for sets, and also for road metal, but they are inferior to the tougher materials, and roads metalled with them are muddy in wet, and very dusty in dry weather." l