2G THE BOTTOM OF THE SEA. 



crust is very far from being uniform. Its maximum 

 cannot exceed, even if it reaches, sixty miles, or less 

 than the sixtieth part of the earth's radius. In some 

 places it is certainly very much less. In the neigh- 

 bourhood of volcanoes, for example, it is so thin that 

 combustible matters are ejected through the fissures 

 in which these mountains abound. 



The greatest depth of the liquid envelope is 

 probably less than six miles, and the gaseous at- 

 mosphere, so far as it is respirable, can hardly be 

 said to reach five miles in height. It is in this 

 limited zone, of ten or twelve miles' thickness, that all 

 the phenomena of life take place. How small is this 

 space compared with the great mass of the globe ; 

 and, to follow out the contrast, what an atom is man 

 compared with the immensity of the universe ! 



In the section (fig. 7) the bed of the great Equi- 

 noctial Ocean, and that of the Indian Ocean, is 

 marked by a dotted line, the data being insufficient 

 to determine their depths with precision. This sec- 

 tion cuts the northern part of South America, and 

 touches the Pichincha volcano. It asses by the 

 Galapagos Islands, which are separated from the 

 continent by a deep arm of the sea. Traversing 

 the middle of the Pacific Ocean, it cuts the archi- 

 pelago of the Scarborough Isles ; and, farther on, the 

 Moluccas, the island of Borneo ; Sumatra, with one 



