THE ROCKS AND THEIR STORY 7 



the Isle of Wight. We should find them in Cornwall, 

 Wales, and Scotland ; and, if we could go deep enough, 

 we should find some such rock as granite underneath the 

 other rocks all the world over. The other rocks, such as 

 the sandstones and clays, are called Sedimentary rocks, 

 because they are formed of sediment, material carried by 

 the sea and rivers, and dropped to the bottom. They 

 are also called Stratified rocks, because they are formed 

 of Strata, i.e., beds or layers, as we see in cliff and 

 quarry. 



But we have seen another kind of rock, the limestones. 

 In Sandown Bay towards the Culvers, bands of limestone 

 run through the dark clay cliffs, and broken fragments lie 

 on the shore, looking like pieces of paving stone. 

 Examining these we find that they are made up of shells, 

 one band of small oysters, the others of shells of other 

 kinds. You see how they have been made. There has 

 been an oyster bed, and the shells have been pressed 

 together, and somehow stuck together, so that they have 

 formed a layer of rock. They are stuck together in this 

 way. The atmosphere contains a small quantity of 

 carbonic dioxide, and the soil a larger quantity, the result 

 of vegetable decomposition. Rain water absorbs some 

 of it, and carries it into the rocks, as it soaks into the 

 ground. This gas has the property of combining with 

 carbonate of lime, the material of which shells and 

 limestone are made. The bicarbonate of lime so formed 

 is soluble in water, which is not the case with the simple 

 carbonate. Water containing carbonic dioxide soaking 

 into a limestone rock or a mass of shells dissolves some of 

 the carbonate of lime, and carries it on with it. When it 

 comes to an open space containing air, some of the 

 carbonic dioxide is given off, leaving the insoluble carbonate 

 of lime again. So by degrees the hollows are filled up, and 

 a solid layer of rock is formed. Even while gathering in 

 the sea the shell-fragments may be cemented by the 



