CONTINENTS; OCEANS 195 



hundred and twenty miles. In 1600 it was only eight miles 

 in circumference. It is now much smaller, being a little 

 over a mile long. 



213. The Location of Continents in Relation to History. 

 Why the lands of the earth are mainly north of the 

 equator, and why there are land projections to the southward 

 with broad expanses of ocean between, may never be known. 

 If the unoccupied northern lands were in the temperate 

 latitudes of the Pacific Ocean, their climate, the history of the 

 peoples of the earth and their occupations, would have been 

 vastly different. The joining of two continents which are 

 now separated, or the separation of two now joined, would 

 have resulted in great differences in the history of mankind. 

 The spread of peoples, the conquest of races, and the devel- 

 opment of industries would all have been different. 



214. The First Oceans. That the first oceans were 

 not pure water is known from the great variety of salts 

 dissolved in the ocean. These salts were probably formed 

 by the chemical action of acid waters on the metals of the 

 rocks. 



215. The Composition of Sea Water. After continents 

 were formed, rivers carried material, both dissolved and in 

 solid form, into the oceans. To-day every soluble mineral 

 substance known in the solid part of the earth is to be found 

 in the sea water. One of the largest constituents of sea 

 water is common salt, which forms 2J per cent of the weight 

 of the sea water; that is, from 1,000 grams of sea water 25 

 grams of salt can be obtained. The sea water also contains 

 much dissolved air, which is necessary for the sea animals 

 that breathe by gills. Spray tossed from the waves catches 

 air and carries it back to the sea. 



216. The Depth of the Ocean. The average depth of 

 the ocean is two and one half miles. The greatest depth is 

 about twice as much, while along the shores of the conti- 

 nents a child may sometimes wade safely many rods from 



