XII.] EARTHQUAKES AND VOLCANOES. 189 



unfrequented districts, which are never recorded in such 

 reports. The total effect of such disturbances upon the 

 surface of the earth must consequently be far from insig- 

 nificant, even in the course of a single year. 



Subterranean disturbances which commence merely with 

 quakings of the ground often terminate with the forcible 

 ejection of heated matter from the interior of the earth. 

 A rent may be produced at some weak point, and this crack 

 then serves for the passage of large volumes of steam and 

 other vapours, with showers of red-hot ashes, accompanied 

 or followed by streams of molten rock. The solid materials 

 are shot forth into the air, and fall in showers around the 

 mouth of the orifice; where they form, by their accumulation, 

 a cone-shaped mound or hill. Such a hill is called a volcano^ 

 or popularly a " burning mountain." It must be borne in 

 mind, however, that it does not " burn," in the sense in 

 which a fire burns, but it merely offers a channel through 

 which heated matter is erupted from below. It differs again 

 from an ordinary mountain, 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 mterior 

 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 round the orifice in conical layers, each forming a 

 mantle thrown irregularly over the- preceding layer, and 

 sloping in all directions 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 



1 Volcano, \\.dX\2cavuhano, from Vulcan, the Roman god of fire. 



