Rocks and Their Origins

by Grenville A. J. Cole

Zircon. Zirconium silicate, ZrSiO₄.

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.

Limestones divide themselves into types produced by chemical precipitation and those due to the accumulation of the hard parts of organisms; but in many of the latter types chemical precipitation also plays a part. Organic action, moreover, frequently promotes the deposition of the chemical types. Detrital limestones, that is, limestones formed from the debris of older ones, are comparatively unimportant. They occur in certain zones of the Chalk and of the Carboniferous Limestone in our islands, and record the breaking up in shallow water of beds that had already become consolidated. The Miocene Nagelfluh conglomerates of the north side of the Swiss Alps are often formed of pebbles of the far older Mesozoic limestones. Similar conglomerates, cemented by calcium carbonate, are now being formed in the river-beds of the limestone karstland of Hercegovina. Limestone, however, as a rule goes to pieces before the buffetings sustained by mixed rocks on a shore. Even if it survives for a time in gravels, percolating waters ultimately dissolve it, and only a porous skeleton, formed of its impurities, remains.

LIMESTONES DEPOSITED FROM SOLUTION

Though calcium carbonate is far less soluble than calcium sulphate, large quantities are carried invisibly, owing to the presence of carbon dioxide, in river waters, and thus accumulate in inland seas that have no outlet except by evaporation. Here Calcareous Tufa may be deposited as a crust upon the shores and on the growing islets, as the water shrinks away, and before the more soluble gypsum and rock-salt can separate out. Hot springs of volcanic origin, like the Sprudel of Karlsbad in Bohemia, may deposit calcium carbonate as the water cools and is relieved from pressure. At Karlsbad, little grains of granite, or of the minerals of granite, serve as centres, and encrusting layers are formed round them, until pea-like bodies are produced. These become cemented together, giving rise to the well-known freshwater pisolitic limestone or roestone.

On the shores of the Great Salt Lake of Utah, calcareous tufa occurs also in the form of grains resembling little eggs. These are the oolitic grains that were first known as constituents of fossil limestones. The calcium carbonate of oolitic grains at Karlsbad, from the Great Salt Lake, and from the sea, is deposited in a form that gives the reaction of aragonite when boiled in cobalt nitrate. A. Lacroix, however, finds that the material at Karlsbad has a specific gravity lower even than that of calcite, and that its double refraction is also distinctly weaker. He has called this form of calcium carbonate "ktypeite."

Travertine is a tufa laid down on twigs and other vegetation, where springs emerge laden with calcium carbonate. In a massive form, it builds tufa-basins, as in the Mammoth Hot Springs of the Yellowstone Park. Both here and at Karlsbad, it appears that vegetation of humble type, multiplying under warm conditions, materially assists the deposit by withdrawing carbon dioxide from the water. The unstable calcium bicarbonate is thus converted into the carbonate, which is thrown down as a quickly increasing crust.