As the distribution of the seas has changed imperceptibly 

 through time, so has their composition. The primeval seas became 

 salty as streams and rivers feeding them washed and dissolved 

 minerals from the land. Although common salt is the main dis- 

 solved material in sea water, many other compounds and elements 

 exist in varying proportions. Since the volume of the sea is so 

 great — 330 million cubic miles — the total weight of valuable metals 

 (although they are present in very small concentrations) exceeds 

 that to be found on the land. Industries thrive on the bromine 

 mined from sea water, and fish grow fat and healthy with the help 

 of small amounts of cobalt, nickel, and vanadium that are circu- 

 lated from the sea-bed sediments by the ponderous underwater 

 movements that form the ocean currents. 



From the early days of oceanographic studies we have known 

 that the salinity of sea water varies from place to place, and from 

 shallow to deep water in any one location. As we shall find in the 

 last chapter of this book, salt is one of the markers which tell the 

 oceanographer where the currents are flowing, but the sea is not 

 well stirred ; each individual current, like the famous Gulf Stream, 

 keeps its entity for thousands of miles, although a little mixing 

 occurs at the fringes. But the body of water moves on, labeled by 

 its salinity and its temperature until it loses its identity in the 

 shallow seas. In general, cold, salty water sinks and warm water, 

 together with fresh water from rivers, floats on top. These different 

 streams of water circulate round the oceans. Not only do they affect 

 navigation, but their continual stirring and upwelling bring the 

 necessary nutrients for animal life up from the nutrient-rich deep 

 layers. In the relatively still subtropical areas there is usually a 

 dearth of plankton — the basic food of larger fishes. There are few 

 plankton because there are few fresh chemicals brought to the 

 surface layers. 



Because the surface of the oceans is exposed to the air and to 

 the warmth of the Sun's rays, evaporation takes place. However, 

 since the oceans are so deep, and because water has a large capacity 

 for holding heat, the temperature of the water stays more or less the 

 same. But in inland lakes like the Dead Sea evaporation leaves 

 great concentrations of dissolved salts, and eventually thick de- 

 posits of valuable materials containing sodium, potassium, iodine, 

 and other elements are laid down. A similar process sometimes 

 takes place near shore when deposits of mud form shallow pools, 

 the water of which alternately evaporates and is replenished. In 

 such cases salt deposits are interleaved between the clays and 

 limestones that are formed. 



In the deep oceans the deposits are different. Sometimes shallow 

 water material is carried far out onto the abyssal plains of the deep 

 ocean floor by turbidity currents which tumble as underwater 

 avalanches down the continental slopes, but the general picture is 

 one of slow and steady settlement of minute, solid particles from 

 the water. This rain of material includes the skeletal remains of 

 small animals, pebbles carried by ice or by floating tree trunks, 

 fragments of pumice from volcanic outbursts, and dust from outer 

 space. The rate of accumulation of this material on the sea bed is 

 unimaginably slow. 



Age after age such material has been filtering down continu- 

 ously to the sea floor. Once it is encased in sediments, which can 



At Batz, near St. Nazaire, Brittany, sea 

 water is pumped into large enclosures called 

 "pans," where it is allowed to evaporate. 

 Ttie remaining salt is then raked into piles. 



Within historical times the land has gained 

 over the sea In the Persian Gulf area. Over 

 the years the Tigris and Euphrates rivers ' 

 have altered the coast line by carrying 

 sediments far out into the gulf. Coast 

 line positions since 3000 B.C. are shown 

 according to various authorities. 



•Baghdad 



Hofuf, 



^' , 3000 B.C. 1 Positions of the coast line at the 



,; y 1200 B.C I head of the Persian Gulf as 



200 6.C J given . by various auttiorities ■ 



Sites of ancient towns 



M^ern towns I 



.)(;. 



SO" 



T 



