=Rock-Salt.= Sodium chloride, NaCl. =Rutile.= Titanium dioxide, TiO_{2}, crystallised in the tetragonal system. _Anatase_ has the same composition, and is tetragonal, but has different fundamental angles. =Serpentine.= Hydrous magnesium iron silicate, H_{4}(Mg, Fe)_{3}Si_{2}O_{9}. =Siderite.= Iron carbonate, FeCO_{3}. =Sillimanite.= See Andalusite. =Talc.= Hydrous magnesium silicate, H_{2}Mg_{3}(SiO_{3})_{4}. =Tourmaline.= A borosilicate of aluminium with various other elements, R′_{9}Al_{3}(BOH)_{2}Si_{4}O_{19}. R represents H, Na, Al, Mg, Fe. =Tridymite.= Silica, SiO_{2}, crystallised in doubly refracting six-sided plates. Its specific gravity is 2·3, that of Quartz being 2·65. =Wollastonite.= See Pyroxenes. =Zeolites.= A series of hydrous aluminium silicates, with potassium, sodium, calcium, and sometimes barium. =Zircon.= Zirconium silicate, ZrSiO_{4}. CHAPTER II THE LIMESTONES INTRODUCTION The term Limestone covers, by common consent, rocks consisting mainly of calcium carbonate. Dolomite (properly Dolomite-rock), in which half or nearly half the molecules consist of magnesium carbonate, is, however, generally included. The convenience of limestones as building materials has given them a world-wide interest. Their stratified and jointed structure appealed to the early Egyptian architect, when he sought blocks for his pyramids. The ease with which limestones could be carved, combined with a reasonable resistance to decay, gave them a pre-eminence with the designers of our rich cathedrals. The Romans found in the stained and altered varieties colour-schemes for basilicas and baths, and their luxurious taste in limestone has been inherited by the modern builders of hotels. The rock suffers, however, from its solubility in water containing even a mild acid. In the gases dissolved by rain-water from the atmosphere, carbon dioxide assumes a far larger proportion than that which it possesses in the air itself. The surface of limestone slabs becomes in consequence pitted and corroded by every rain that falls. The sulphuric acid in the air of modern coal-consuming cities is, however, still more deadly in its action. J. A. Howe, in his recent work on building stones, is of opinion that limestone is unsuitable for towns. Limestones may broadly be recognised by their solubility in cold dilute acids, with brisk evolution of carbon dioxide. Dolomitic varieties require hot acid.