THE STUDY OF THE ROCKS 



and their shells have fallen to the bottom of their 

 watery home. Now we know their shell remains as 

 chalk or if it has undergone great pressure, owing 

 to changes in the earth's surface, as limestone. 

 There are rock masses also formed of the remains 

 of countless Diatoms; sponge spicules, too, have 

 played their part in rock formation. 



It is clear from what we have written that, in 

 the first place, the rocks of the earth form two 

 great divisions, the rocks which formed the original 

 crust of the earth, Primary rocks they are called, 

 and the later-formed Secondary rocks. The Primary 

 rocks are often glassy in appearance, they show un- 

 mistakable signs of having once been molten and they 

 never contain animal or plant remains, fossils as they 

 are called, for the reason we have already explained. 

 The secondary rocks often betray their origin by 

 occurring in layers, or strata to speak more scienti- 

 fically. Always when we think of the rocks we must 

 think in thousands of years, then it will be easier 

 to understand the formation of these strata, each 

 one of which may represent the work of hundreds 

 and hundreds of years. 



Now to return to our sand; we have explained 

 briefly that sand is really the remains of broken up 

 rocks. First we have the solid rock, boulders are 

 broken from it as the boulders are acted upon by 

 rain, frost and wind : they become more and more 

 broken up and at the same time they are carried 

 towards the sea where they take the form of shingle. 

 The shingle, in the course of time, becomes broken 



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