1 62 THE WORLD'S LUMBER ROOM. 



foot thick in the middle. As much as 216,000,000 pounds 

 were obtained one year, for one of the lakes is forty miles wide. 



Supposing communication were cut off between the Black 

 Sea, Mediterranean, and Atlantic, then as the water removed 

 by evaporation would exceed that received by the rivers, the 

 seas would become salter and salter until the water was 

 saturated, after which all the salt which could not be held 

 in solution would simply fall to the bottom and there accu- 

 mulate. Something of this sort has probably taken place 

 in the Dead Sea, which has nowhere any visible outlet.* 



The salt we use with our food is manufactured from 

 the Cheshire and Worcestershire brine-springs, which have 

 flowed through beds of salt and are in many cases almost 

 saturated. They are known to have flowed for 1,000 years, 

 and as 1,630,000 tons of salt have been obtained in one 

 year from the Cheshire springs alone, they must have con- 

 veyed enormous quantities to the Mersey, and so back to 

 the sea. Much of the ChesTrire rock-salt is so pure as to 

 need nothing but crushing before it is fit for use. 



Another mineral often found with rock-salt is gypsum, or 

 sulphate of lime, which is the first to separate and fall to the 

 bottom when sea-water is evaporated. Beds of salt and 

 gypsum two feet thick have been formed on the rocks in 

 some places simply by the evaporation of spray. 



Gypsum, when burnt, and thus freed from water, is called 

 plaster of Paris ; this is made into a paste with water and 

 used for making casts and moulds. 



If all the invisible salts of the sea could be extracted, 

 they would cover several millions of square miles one mile 



* Sea-water is not nearly saturated. 



