Earthquakes and Volcanoes 



below. It differs again from an ordinary moun- 

 tain, in that it is simply a heap of loose materials 

 and melted matter, which has been piled up 

 layer after layer, around a hole leading down to 

 the interior of the earth. Hence, if a volcano 

 were cut through, it would probably present a 

 section something like that shown in Fig. 50. 

 Here a channel, a, has been opened through 

 strata, b, b, originally horizontal, and the ejected 

 matter has fallen all around the orifice in conical 

 layers, each forming a mantle thrown irregularly 

 over the preceding layer, and sloping in all direc- 

 tions away from the central chimney. 



At the mouth of the volcanic pipe, there is 

 usually a funnel-shaped opening known as the 

 crater. Fragmentary materials falling back 

 into this cup, or rolling in from the sides, form 

 layers which slope towards the vent and there- 

 fore in the opposite direction to the dip of the 

 volcanic beds which make up the mass of the 

 mound. A section of a cone of loose cindery 

 materials is given in Fig. 51, and shows the dif- 

 ference of dip just referred to. The molten 

 matter which wells up the throat of a volcano, 

 cements the loose ashes and cinders into a com- 

 pact mass, where it comes in contact with them, 

 and thus forms a hard stony tube lining the vol- 

 canic chimney. 



At the beginning of an eruption, clouds of 

 steam are copiously belched forth, showing that 

 water has its part to play even in these fiery phe- 

 nomena, The steam generally issues spasmodic- 

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