6o SECRETS OF EARTH AND SEA 



every succeeding eruption, and now we have the records 

 of photography. 



The crater or basin formed by a volcano starts with 

 the opening of a fissure in the earth's surface communica- 

 ting by a pipe-like passage with very deeply-seated molten 

 matter and steam. Whether the molten matter thus 

 naturally " tapped " is only a local, though vast, accumula- 

 tion, or is universally distributed at a given depth below 

 the earth's crust, and at how many miles from the surface, 

 is not known. It seems to be certain that the great 

 pressure of the crust of the earth (from five to twenty-five 

 miles thick) must prevent the heated matter below it 

 from becoming either liquid or gaseous, whether the heat 

 of that mass be due to the cracking of the earth's crust 

 and the friction of the moving surfaces as the crust cools 

 and shrinks, or is to be accounted for by the original high 

 temperature of the entire mass of the terrestrial globe. 

 It is only when the gigantic pressure is relieved by the 

 cracking or fissuring of the closed case called " the crust of 

 the earth " that the enclosed deep-lying matter of immensely 

 high temperature liquefies, or even vaporizes, and rushes 

 into the up-leading fissure. Steam and gas thus " set 

 free " drive everything before them, carrying solid masses 

 along with them, tearing, rending, shaking " the foundations 

 of the hills," and issuing in terrific jets from the earth's 

 surface, as through a safety valve, into the astonished 

 world above. Often in a few hours they choke their own 

 path by the destruction they produce and the falling in of 

 the walls of their briefly-opened channels. Then there is 

 a lull of hours, days, or even centuries, and after that 

 again, a movement of the crust, a " giving " of the blockage 

 of the deep, vertical pipe, and a renewed rush and jet of 

 expanding gas and liquefying rock. 



The general scheme of this process and its relations 

 to the structure and properties of the outer crust and 



