_Invaluable_, extremely valuable. For what is it used? Slate has superseded the use of lead for covering roofs, even of the largest buildings; being lighter and more durable, it is preferable to tile: it is also employed for slabs to form cisterns, shelves for dairies, and other purposes, on account of its strength, coolness, and the ease with which it can be cleaned; the latter quality renders it also of great value in the business of education, as a cheap substitute for paper. The ancients were unacquainted with the use of slate. What other kinds of stone are used in building? Limestone, or the calcareous rocks of the geologist: of these there are many varieties. Those which are easily cut and polished are termed marbles, and are used in sculpture and in ornamental architecture. The coarser marbles are used for the common purposes of building. _Calcareous_, partaking of the nature of calx or lime,--a term employed to describe chalk, marble, and all other combinations of lime with carbonic acid. _Geologist_, one who studies the science of Geology. Of what do Calcareous Earths or Stones consist? Calcareous earths, stones, or rocks consist of lime, or pure calcareous earth, carbonic acid, and water. What is Quick-Lime? Limestone deprived of its carbonic acid and water by being subjected to an intense heat in a kiln. How are these Stones wrought? To whatever purpose the stones are to be applied, the larger blocks obtained from the quarry must be cut into smaller and more manageable pieces by sawing: the saw used is a long blade of steel, without teeth, fixed in a heavy wooden frame. These huge saws are worked by one or two men who sit in boxes to shelter them from the weather; water is caused to drip constantly into the cut, to facilitate the motion of the saw, and keep it cool, so as to prevent it from losing its temper. _Huge_, very large. _Temper_, hardness; in speaking of metals it signifies the state to which they are reduced, especially with regard to their hardness. What is Steel? Iron combined with a small portion of carbon; its chemical name is _Carburet of Iron_. It is not so malleable as iron in its ordinary state; but is much harder, more elastic, and susceptible of a higher polish. Of this material are manufactured knives, swords, and all kinds of cutting instruments and edge tools, used for domestic purposes and in the arts, from the ponderous pit-saw to the finest lancet. Good steel is much more ductile than iron; and a finer wire may be drawn from it than from any other metal. The excellence of edge-tools depends upon their temper. _Ponderous_, heavy. You say that a Geologist is one who studies Geology: what is meant by this term?