THE EARTH'S INTERIOR. 231 



we sliall see that the liglitest materials in its com- 

 position are just where we should expect them to 

 be — on the outside — and the heaviest, on tlie 

 other hand, are just where we might naturally 

 look to find them — at the bottom and near the 

 earth's centre. On the very exterior of all, sur- 

 rounding our globe like a thick but light envelope, 

 comes a deep layer of gaseous matter, the air or 

 atmosphere, thinner and lighter as we rise towards 

 the top, on mountain-summits or in an inflated 

 balloon, and denser and heavier near the solid sur- 

 face or at sea-level. Next to this outer gaseous coat 

 comes a more partial envelope, the water of the 

 ocean, collected into the profounder hollows of 

 the crust, heavier than the air, but lighter than 

 the rocks and soil which form the solid tertiary 

 layer. This solid tertiary layer itself, we may 

 conclude, is, in the same way, lighter and less 

 dense than the yet deei)er inside ; for, when the 

 whole mass was still liquid and molten, and the 

 ocean existed only on its face in the shadow}' form 

 of steam or vapor, it is natural to suppose that 

 the heavier materials, such as lead and mercury, 

 would sink, for the most part, steadily towards 

 the centre, under the influence of gravitation, 

 while the lighter, which compose in the main the 

 existing crust — largely silicious in character — ■ 

 would float on to}) like oil on water. Thus, we 

 get a clear mental picture of our earth as a solid, 



