THE ROCKS AND THEIR STORY 5 



of the interior are intensely hot. The temperature in a 

 mine becomes hotter, about iF. for every 6oft. we go 

 down on the average. We know that there are great 

 quantities of molten rock in places, which, in a volcanic 

 eruption is poured out in sheets of lava over the land. 

 There are great quantities of water turned into steam by 

 the heat, and in an eruption the steam pours out of the 

 crater of the volcano like the clouds of steam out of the 

 funnel of a locomotive. The people who live about a 

 volcano are living, as it were, on the top of the boiler of 

 a steam engine ; and their country is sometimes shaken 

 up and down like the lid of a kettle by the escaping steam. 

 In such a country the land is often changing its level. 

 A few miles from Naples at Pozzuoli, the ancient Puteoli, 

 may be seen columns of what appears to be an ancient 

 market hall, though it goes by the name of the Temple 

 of Serapis. About half way up the columns are holes 

 bored by boring shellfish, such as we may find on the shore 

 here at low tide. We see from this that since the building 

 was constructed in Roman times the land has sunk, and 

 carried the columns into the sea, and shellfish have bored 

 into them. Then the land has risen, and lifted the columns 

 out of the sea again. 



But it is not only in the neighbourhood of volcanoes 

 that the land is moving. Not suddenly and violently, 

 but slowly and gradually great tracts of land rise and sink. 

 Sometimes the land may remain for a long time nearly 

 stationary. The Southern coasts of England seem to 

 stand at much the same level as in the time of the 

 Romans 1,500 or 2,000 years ago. On the other hand there 

 is evidence which seems to show that the coast of Norway 

 has for some time been gradually rising. 



It was thought at one time that the interior of the 

 earth was liquid like molten lava, and that the land we 

 see was a comparatively thin crust over this like the crust 

 of a pie. But it is now believed for various mathematical 



