164 ELEMENTS OF SCIENCE 



cataract a mile wide, over a precipice fifty feet high 

 into the ocean. 



Volcanoes are very unequally distributed over the 

 earth's surface. One of the most considerable volcanic 

 regions is that in the Andes (between Quito and Chili), 

 and there is another in Mexico. A volcanic region of 

 very great extent passes from the Philippines through 

 the Moluccas, Timor, Lombok, and Bali into Java and 

 Sumatra. Volcanoes have been known to resume their 

 activity after a quiescence of centuries. 



The slow upheaval and depression of different parts of 

 the earth's surface have been proved by direct observa- 

 tions. The Andes have been rising century after century 

 at the rate of several feet, while the region of the eastern 

 Pampas has risen but a few inches. The land of Scan- 

 dinavia, towards North Cape, rises above five feet in a 

 century, and very many other instances could easily be 

 adduced of slow secular elevation. Soundings often give 

 good reason to suppose both that some rather distant lands 

 once formed part of an adjacent continent, as also that 

 islands, which by their proximity to some mainland 

 might be supposed to have been previously united to it, 

 have not really been so, but have only become nearer to 

 it through a recent elevation of the mainland's coast. 

 But however considerable such changes may here and 

 there have been, it seems probable that the great oceans 

 and continents have been permanent save for more or 

 less considerable modifications of their margins and 

 boundaries. 



The science which treats of the structure of the earth, 

 and the causes which have brought about the present 

 condition of its surface, is the science of geology. The 

 earth's crust, mainly composed of the mineral substances 

 before noticed, is made up of superimposed masses of 



