Geography of Ohio 373 



secondary rock. Many of the constituents of older strata are 

 broken up into their chemical elements, and as such enter the 

 oceans. Because of this fact, ocean water is salty and heavier 

 than fresh water. We sometimes speak of the saltiness of the 

 sea, usually referring to sodium chlorid. But this is a single 

 chemical ; all the list of chemicals found in rocks exists in sea water. 

 Certain animals in the oceans feed upon particular chemicals. 

 The carbonates, and to some extent the silicates, are converted 

 into the hard parts of sea organisms. When these organisms die, 

 their shells, spines, etc., are assembled at the bottom, and when 

 especially numerous make characteristic deposits; becoming rock, 

 these are named in accordance with their prevailing constituent. 

 If the animal's remains consist largely of carbonates, the result- 

 ing rock is called a limestone; if the clayey or sandj^ content is 

 conspicuous, it may be called a calcareous shale or sandstone. 

 In the deeper parts of the seas, however, smaller organisms exist, 

 and their remains become a part of the ooze that mantles the bot- 

 tom of the ocean basins. Rocks which are built up largely of the 

 decayed products of life are called organic. 



Rocks precipitated from solution. Sometimes the percentage 

 of a given chemical becomes so high in water that it is precipi- 

 tated, depositing on the bottom of the basin that particular salt. 

 It may be a gypsum deposit, a sodium chlorid deposit, or some 

 carbonate. This process of direct precipitation is usually asso- 

 ciated with arid conditions in an inland drainage basin. Here 

 the evaporation is greater than the rainfall; as a result, the amount 

 of chemicals in the water is constantly increasing. When, for a 

 particular compound, the saturation point is reached, it is precip- 

 itated. Later the solution may become saturated for another 

 compound, which in turn is deposited. 



The remarkable salt deposits in Ohio belong to this class of 

 rocks. Their genesis, however, may not be associated with inland 

 drainage; this question is discussed in an earlier section (p. 152). 



THE GROWTH OF LAND AREAS 



The geologic scale of Ohio includes clastic, organic, and chem- 

 ical rocks. We have no crystallines, and no metamorphosed rocks 

 in place; but scattered over the surface throughout at least half 

 of the state are found bowlders of crystalline and metamorphosed 

 material brought from Canada by the glaciers. 



