_THE GEOLOGICAL PART._ You will have learned from other books something about the size and shape of our World: for instance, that it is a great round body, or rather more like an orange, a little flatter one way than the other, and about 8,000 miles through, from one side to the other, and that it turns round once in every twenty-four hours; but I have only to tell you now what it is made of. The material is called rock, earth, or soil; and there are many kinds of it, such as granite, gravel, clay, sand, chalk, mud, and so on; and we shall see that many of these different soils contain different fossils. It is supposed that a very long time passed while these were being laid one upon another, and before many plants or animals lived here, and there are good reasons for thinking that underneath these soils the Earth is very hot, perhaps in a melting state, because we know that volcanoes like Vesuvius and Ætna throw out flame and smoke and lava, which is melted earth and rock; and that this lava has run down the sides of the mountains for miles, in a great stream of liquid material, and covered up and destroyed whole villages and towns. You have heard of earthquakes, when the ground shakes and cracks, and houses are thrown down, as they have often been in Spain, Italy, and South America. This convinces us that the inside of the earth must be very different from the outside. Two or three years ago Mount Vesuvius was boiling up, and the people of Naples feared that it would throw out some of the terrible lava and red-hot cinders, and burn up their vineyards and perhaps injure their city; and during the last two or three years many people have been killed by earthquakes in South America. These things seldom happen in the North of Europe, and when they do they are only slightly felt, and people are not killed, neither are houses thrown down. Still, this shows that there must be some great force underneath us, and very much heat. We see nothing of this when we look upon the green fields, and we should scarcely think it possible if there were not histories about these eruptions, as they are called. But when I tell you that I have felt the Earth tremble, and seen fire rushing out from the top of a high mountain whose sides were covered with snow, you will understand how real it is—though it may seem so strange. People at one time liked to fancy that powerful spirits lived in volcanoes and made them their workshops: but we know better now. Well, the interior of the Earth is evidently very different from the part we live upon; and it is the outside we have to think about now, which would be dreadfully cold if the sun did not shine upon it, though the inside is so hot.