_Tut._ Yes. There are pits of the best kind of it near Woburn in Bedfordshire, and Nutfield in Surrey, England. The different kinds of slate, too, are stones of the argillaceous class; and very useful ones, for covering houses, and other purposes. _Har._ Are writing slates like the slates used for covering houses? _Tut._ Yes; but their superior blackness and smoothness make them show better the marks of the pencil. _Geo._ You have mentioned something of sand and flints, but you have not told us what sort of earths they are. _Tut._ I reserved that till I spoke of the third great class of earths. This is the _siliceous_ class, so named from _silex_, which is Latin for a flint-stone. They have also been called _vitrifiable_ earths, because they are the principal ingredient in glass, named in Latin _vitrum_. _Geo._ I have heard of flint-glass. _Tut._ Yes—but neither flint, nor any other of the kind, will make glass, even by the strongest heat, without some addition; but this we will speak of by-and-by. I shall now tell you the principal properties of these earths. They are all very hard, and will strike fire with steel, when in a mass large enough for the stroke. They mostly run into particular shapes, with sharp angles and points, and have a certain degree of transparency, which has made them also be called _crystalline_ earths. They do not in the least soften with water, like clays; nor are they affected by acids, nor do they burn to lime, like the calcareous earths. As to the different kinds of them, _flint_ has already been mentioned. It is a very common production in some parts, and is generally met with in pebbles, or round lumps forming pebbles, in gravel-beds, and often almost entirely covering the surface of ploughed fields. _Har._ But do they not hinder the corn from growing? _Tut._ The corn, to be sure, cannot take root upon them, but I believe it has been found that the protection they afford to the young plants which grow under them is more than equal to the harm they do by taking up room. Flints are also frequently found imbedded in chalk under the ground. Those used in the Staffordshire potteries chiefly come from the chalk-pits near Gravesend. So much for flints. You have seen white pebbles, which are semi-transparent, and when broken resemble white sugar-candy. They are common on the seashore, and beds of rivers. _Har._ O, yes. We call them fire-stones. When they are rubbed together in the dark they send out great flashes of light, and have a particular smell. _Tut._ True. The proper name of these is _quartz_. It is found in large quantities in the earth, and the ores of metals are often imbedded in it. Sometimes it is perfectly transparent, and then it is called _crystal_. Some of these crystals shoot into exact mathematical figures; and because many salts do the same, and are also transparent, they are called the _crystals_ of such or such a salt. _Geo._ Is not fine glass called crystal, too?