I go 



PHYSIOGRAPHY. 



[chap. 



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 difference of dip just referred to. The molten 

 matter which wells up the throat of a volcano, cements the 

 loose ashes and cinders into a compact mass, Avhere it comes 

 in contact with them, and thus forms a hard stony tube 

 lining the volcanic chimney. 



J 



Fig. 50.— Diagrammatic Section of a Volcano. 



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 phenomena. The steam generally 

 issues spasmodically, each puff giving rise to clouds which 

 shoot up to a great height, and are either dissipated or con- 

 densed in torrents of rain. Associated with the steam are 

 various gaseous exhalations, most of which, however, are not 

 combustible. Hence, the appearance of a column of flame, 

 often said to be seen issuing from a volcano, must generally 



