THE SILURIAN PERIOD 443 



the iron having been brought to the sea by streams which had leached 

 it from the igneous rocks over which they flowed. 



The presence of iron ore, limestone, and coal within short distances 

 of each other near Birmingham, Alabama, has made that city a great 

 center for iron and steel industries. Coal is necessary to reduce the 

 iron, and limestone is used as a flux to carry away the siliceous im- 

 purities. 



Deserts. — During a portion of the Silurian (Salina) in eastern 

 North America the climate was arid and desert conditions prevailed. 

 This is shown by the beds of salt and gypsum, and by the red color of 

 the shales. In New York state 325 feet of solid salt have been pene- 

 trated by wells. These salt beds are lens-shaped, and the conditions 

 under which they were deposited may not have been unlike those 

 to-day in the region of the Caspian Sea, the Dead Sea, and Great Salt 

 Lake, or back of bars as described below. Such an arid climate may 

 have been produced by high lands to the south and east, which shut 

 ofF the moist winds from the Atlantic and the Gulf of Mexico. 



Origin of Rock Salt. — Salt is primarily formed by the evaporation 

 (p. 135) of the salt water of lakes or the ocean, and is accumulating to- 

 day in certain salt lakes which have been greatly concentrated. The 

 evaporation of inland salt lakes does not, however, seem adequate to 

 produce thick beds of pure salt such as occur in certain regions. 



The theory which best explains the origin of massive salt deposits 

 assumes that a body of ocean water had been shut off* partly or com- 

 pletely by a low bar. If the region in which this occurred was arid, 

 the evaporation of the water back of the bar would exceed that car- 

 ried in by the rivers and that derived from the ocean. The lowering 

 of the water of the bay by evaporation would permit the ocean water to 

 flow in if the bar were incomplete ; if, however, the bar were complete 

 and the bay entirely shut off from the ocean, forming a lake, ocean 

 water would enter only during storms or at high tide. In time, the 

 concentration of the water would be so great that common salt and 

 other salts would be precipitated. Under conditions such as those 

 outlined above, pure salt might accumulate to a considerable thickness 

 without the admixture of mud. Occasionally, the purity of the salt 

 might be broken by sheets of mud brought in by streams swollen by 

 the torrential showers of desert regions. 



The Silurian salt of New York seems to have been deposited either 

 in extensive salt lakes or in an arm of the sea which was partially shut 

 off from the sea by a bar. 



