Alluvial, deposited from water.

Of what is this last compounded?

The Alluvial formation is composed of sand, gravel, loam, clay, turf, &c., and contains plants, roots, moss, bones, petrified wood, and skeletons of animals.  It is distinguished from the Tertiary formation chiefly by its superior position, and by extending over regions where existing streams or other causes now in action could have produced it.  Some geologists mention another formation called the Volcanic, because composed of minerals thrown from the crater of a volcano, such as pumice stones, lava, &c.

     Crater, the mouth or opening of a volcano.

     Petrified, hardened into stone.

You mentioned Silicious and Argillaceous Earths:  is not, then, the earthy covering of our globe of one common character?

No; by earth is understood a combination of many distinct bodies.  Chemists, by separating earths from each other, and from foreign matters connected with them, have discovered nine or ten primitive earths; all of these, except silex, are compounds of oxygen with metallic bases.

     Chemist, one who understands the science of chemistry.

Of which of these Simple or Primitive Earths are the solid portions of the globe principally composed?

Of flint or silex, lime or calcareous earth, and clay or argil, in various degrees of combination, the greatest parts of the mountains and plains, and the whole of what we commonly understand by soil, mould, earth, &c. are composed.  These, however, though forming nearly all of the solid portions of the world, are constantly mixed with foreign matters, as metals, (particularly iron,) and acids, (as carbonic acid.)

What are the properties of Silex?

Silex, or pure flint, will not dissolve in water, nor can it be melted by itself in any heat; but combined with alkalies, as soda or potash, it forms glass.  It is the principal ingredient of most of the precious stones.

What are the chief uses of Silex?

It is the most durable article for the formation of roads; a necessary ingredient in earthenware, porcelain, and cements; and the principal material of glass and vitreous substances.  The making of pastes or artificial gems is a branch of the art of glass-making; the basis used is a very hard and pure silex.

     Basis, that part of any mixture which is the ground or
     base; the first principle or element of a substance.

Describe the properties of Lime.

It is of a white color, and possesses a hot, caustic taste.  It forms peculiar salts with acids; changes vegetable blues to green; will not fuse; gives out a quantity of caloric when united with water; and absorbs carbonic acid when exposed to air.  Lime is very useful in the arts and manufactures, in medicine, &c.  The farmers use it as manure to fertilize land.