Rocks and Soil 



297 



concrete the shape desired, molds are used. Ordinarily these are 

 built of wood. The concrete mixture is put in them and lightly 

 pounded or tamped into place. As soon as the material has hard- 

 ened sufficiently the wooden molds are removed. 



In the case of large structures, or of those subjected to heavy 

 strain, additional strength is often given by placing metal rods 

 within the concrete masses. 



Ore and Metals. Metals are among the most common mate- 

 rials used in this scientific age. Iron and steel are employed 

 everywhere in building construction. They also form a large part 

 of most machines. Copper, 

 tin, gold, and silver are 

 familiar to us from their many 

 uses in the modern home. 

 Some metals are found in 

 their natural state either in 

 veins or in cracks of rock 

 masses. Most, however, are 

 found thoroughly mixed with 

 rock-like material. Rock mate- 

 rial containing metal is called 

 ore. 



Gold occurs in veins of quartz and in some gravel deposits. 

 To extract gold from quartz and make it available for use, the 

 quartz must be crushed and then specially treated to remove the 

 gold. Silver is often separated from its ore by electric processes. 



Iron ore, which generally occurs in great masses, is excavated 

 by machinery and shipped to iron or steel mills. Here it is mixed 

 with varying amounts of limestone and coke, according to the 

 quality of the ore. The mixture is dumped into large steel con- 

 tainers lined with fire brick, called blast furnaces. The coke is 

 then fired, the draft being supplied by the forcing in of heated 

 air at the base. During this process, called smelting, the lime 

 from the limestone combines with the impurities of the ore to form 

 slag. The heavy iron, melted by the heat, settles beneath the 

 slag and is drawn off and run into molds to harden. It is then 

 called pig iron. This iron must still be treated by other processes 



Brown Brothers. 



UNLOADING IRON ORE 



