194 GENERAL SCIENCE 



heat and pressure to which it has been subjected. Some of 

 the marble quarried in Italy and Greece has been famed for 

 centuries in art because of its snowy whiteness. It has 

 a texture well suited to the chisel of the sculptor. 



Great caverns are not infrequently met in the limestone 

 strata of the earth's crust, with miles of underground pas- 

 sages, and with vaulted chambers having huge masses of 

 stone in the form of icicles hanging from the roof. Here on a 

 large scale, and through long periods of time, the solvent 

 action upon limestone of water containing carbon dioxide 

 has been operative. The dissolved material has been carried 

 away by underground streams. Where water containing 

 the "lime" has evaporated in dripping from the roof, lime- 

 stone has been left in place either as stalactites above or as 

 stalagmites rising from the floor. These may have become 

 joined into an ever enlarging mass that serves as a pillar 

 to support the roof. The Mammoth Cave in Kentucky, 

 and the Wyandotte Cave in Indiana, are notable examples 

 in the United States of these caverns in limestone rock. 



Some sandstones wet with hydrochloric acid effervesce 

 freely. If acted upon sufficiently long they are reduced to a 

 mass of separated grains of sand by reason of the removal 

 by the acid of the limestone cement. Such sandstone, as 

 well as limestone itself, when used for building purposes is 

 likely to "weather" more or less. It crumbles enough to 

 become unsightly and a menace to the strength of walls. 



Carbonates of copper, of iron, of zinc, and of other metals 

 are valuable ores. From them the respective metals are 

 generally obtained: 



1. By roasting the ore which changes it to an oxide just 

 as limestone (CaCO 3 ) when heated yields calcium oxide 

 ("quick-lime") and carbon dioxide as shown in the equation 

 CaCO 3 = CaO + CO 2 . 



2. By a "reduction" of these oxides of the metals to a 



