52 



NATURE STUDY IN SCHOOLS. 



that slowly rose from the primitive ocean, was granite, an erupted rock. As 

 the barren rock towered above the sea, it was fiercely attacked by the wind 

 and rain, and the powerful, grinding waves. Soon a part of the rock was 

 ground to sand and mud, and then we had the second kind of rocks in the 

 world, sandstone or slate, for sandstone is nothing but hardened and com- 

 pressed sand of the seashore. While this grinding action was going on, 

 there came to be corals and other lime secreting animals, out in deeper wa- 

 ter. These little soft polyp animals had the power of extracting the lime or 

 calcium ( carbonate ) that is in solution in the sea water, and making hard, 

 limy skeletons of it. 



These animals were so numerous that vast quantities of lime rock were 

 thus built up. These three rocks, the granite, an erupted rock, the sand- 

 stone, an aqueous or stratified rock, and the limestone or marble, an organ- 





Fig. 25. 



7 ''^T -- 





J<: '"' v! h^V* vv \^ °^^ 





A, Stratified rock; B, granite mountain from which the stratified rocks came. 



ic rock, are the principal rocks, composing the earth's crust. All others 

 may be considered as modifications of them. A good way to understand 

 stratification is to fill a bottle half full of sand and pebbles, pour water in 

 till the bottle is full, then close and shake violently. On resting, the peb- 

 bles at once sink to the bottom of the bottle, the larger grains of sand 

 next, then the smaller grains, and lastly, the very fine sand and mud will 

 settle. This happens at the seashore. At Crescent beach for example, we 

 first walk down a banking of rocks and boulders, then we come to the peb- 

 bles on the beach, next, and nearer the water, we have sand, and out in deep 

 water, mud. 



Geologists are thus able to trace out ancient beaches, and if they find, 

 for instance, some very fine grained slate, they conclude that it was formed 



