1 THE PROTOZOA 11 



Such a deposit, owing to the continuous pressure of the 

 ocean above, gradually becomes consolidated into a hard chalk 

 rock. In many cases, after long ages, it has happened that 

 such a rock has been raised above the sea-level by the 

 gradual shifting in relative position of land and sea, and then 

 it may form a white chalk cliff or down, such as those to be 

 seen along many parts of our coasts.^ 



Such chalk rock, if it should be exposed to extreme heat 

 as well as pressure, such as might be produced by volcanic 

 action, becomes crystalline, and the rock known as marble is 

 formed. 



It is wonderful to think of the constant circulation of material 

 that is always going on, and resulting in a continual change 

 of the face of the earth. The rain drives down through the air, 

 dissolving as it descends some of the very soluble gas, carbon 

 dioxide, which, owing to the respiration of all living things, 

 is always present in the air. By virtue of the carbonic acid 

 so formed, the water, as it trickles through the earth, is able to 

 dissolve little particles of chalky material (calcium carbonate), 

 and other solid substances present in the earth, and even to 

 wear awayhard solid rock. Finally the water with its dissolved 

 matter may find its way into a river, and be carried far out 

 to sea. Here the sun's rays beat down on the surface of the 

 water and draw the water-drops up again out of the ocean 

 into the air, changing them from visible form to invisible, 

 from liquid to gas or vapour. But it is the pure water alone 

 that rises in the air ; all that it carried has to be left behind, 

 and so the sea accumulates an increasing amount of dissolved 

 matter, and consequently tends to get Salter and Salter. The 

 little, soft-bodied, unicellular organisms, however, which live 

 in this salt water, need to make for themselves protective 

 cases, and they have learnt to absorb the salt water and take 

 from it some of the dissolved materials, from which the living 

 protoplasm of their bodies is able to build up their little shells- 

 of chalk or silica, and in so doing they help to prevent the 

 water from getting too dense for animals to live in. Then, 

 as explained before, these shells accumulate for thousands 

 of years, and form the chalk rock, which may again form dry 

 land, and again, by the action of water, be worn away and 

 carried back to the sea ; — and so the cycle goes on. 



' See Huxley on " A Piece of Chalk,'' Oollected Essays, vol. viii. 



