5 o ELEMENTARY SCIENCE 



bodies of plants, probably in swamps. These accumula- 

 tions subsequently underwent changes which were largely 

 due to pressure. Hard coal (anthracite} has been sub- 

 jected to greater pressure than soft coal (bituminous). 



4. The outer part of the earth is largely composed of 

 layers called strata (singular, stratum}. Its stratified nature 

 may be observed in many mountain regions where the 

 upfoldings are sharp and broken. Sometimes it may be 

 seen on the sides of railway cuttings, and always, when 

 deep wells are dug, the boring passes through different 

 layers, as of clay, gravel, and rock,* of which there are 

 many varieties. 



The early geologists were much puzzled by these strata 

 and brought forward many interesting theories as to their 

 cause. But as knowledge grew, it became easy to under- 

 stand that they are due simply to the surface movements 

 and water work that you have been considering. For 

 example, a layer of deep mud might have formed over the 

 bottom of a shallow sea, not far from the land. Then 

 by a shift of the surface (a slow and gentle earthquake) 

 this region may have become deep sea, far from land. 

 The wash of the land may have reached it no more, but 

 the shells of tiny marine animals, for long ages, accumu- 

 lated on top of the mud. Then, due to another shift, 

 came a period in which sand was the chief thing laid down. 

 So these changes went on. Due to the pressure of new 

 layers, and to chemical and other physical changes, the 

 older layers became hardened into rock. Then at last 

 there may have come a great disturbance, due to an adjust- 



Rock means any hard part of the earth's crust. It does not need to 

 be broken. When you say you pick up a rock, it would be more accurate 

 to say a piece of rock, 



