Har. But most sand is broken or yellowish.
Tut. That is owing to some mixture generally of the metallic kind. I believe I once told you that all sands were supposed to contain a small portion of gold. It is more certain that many of them contain iron.
Geo. But what could have brought this quartz and crystal into powder, so as to have produced all the sand in the world?
Tut. That is not very easy to determine. On the seashore, however, the incessant rolling of the pebbles by the waves is enough in time to grind them to powder; and there is reason to believe that the greatest part of what is now dry land was once sea, which may account for the vast beds of sand met with inland.
Geo. I have seen some stone so soft that one might crumble it between one’s fingers, and then it seemed to turn to sand.
Tut. There are several of this kind, more or less solid, which are chiefly composed of sand conglutinated by some natural cement. Such are called sandstone, or freestone, and are used for various purposes, in building, making grindstones, and the like, according to their hardness.
Har. Pray, what are the common pebbles that the streets are paved with? I am sure they strike fire enough with horses’ shoes.
Tut. They are stones of the siliceous kind, either pure or mixed with other earths. One of the hardest and best for this purpose is called granite, which is of various kinds and colours, but always consists of grains of different siliceous stones cemented together. The streets of London are paved with granite brought from Scotland. In some other stones these bits of different earths dispersed through the cement are so large as to look like plums in a pudding; whence they have obtained the name of pudding-stones.
Geo. I think there is a kind of stones that you have not yet mentioned—precious stones.
Tut. These, too, are mostly siliceous; but some even of the hardest and most valuable are argillaceous in their nature, though possessing none of the external properties of clay. The opaque and half-transparent precious stones, such as jasper, agate, cornelian, and lapis lazuli, are engraved upon for seal-stones; the more beautiful and transparent ones, as ruby, emerald, sapphire, topaz, which go by the name of gems, are generally only cut and polished, and worn in rings, ear-rings, necklaces, and the like.