ORIGIN AND STRUCTURE OF VOLCANIC CONES. 779 



ance at the surface, and in fact may have parted company with the 

 lava at very considerable dei3ths. 



Or the latter may have been forced up almost simultaneously with 

 the vapor, and poured out over the edge of the primitive cone. This, 

 however, is not the general rule, for an escape of much gaseous ma- 

 terial nearly always precedes for a variable period the appearance of 

 the lava. In fact, when a volcanic outburst has forced a convenient 

 passage for the vapor, the exit of liquid rock seems of secondary im- 

 portance, for generally the terrific explosions, earthquakes, and sub- 

 terranean thunder that accompany the first stage of eruption are more 

 or less absent, or at least much diminished during the welling up of 

 the fluid rock. If, as in the latter case, a cone of some considerable 

 size has been formed, the lava will rise and occupy the whole of the 

 crater-cavity. Two things may happen : If the cone which now 

 forms, as it were, an embankment around the lava is of suflicient 

 strength to withstand the pressure of the fluid mass contained within 

 it and the continual explosive vibrations, the liquid rock pours out 

 over the edge of the crater down the side of the cone, and may con- 

 tinue its course for variable distances from its starting-point ; or if, 

 on the other hand, the cone is too weak to support the strain, it may 

 break away and give free passage to the lava through the breach. 

 This condition is well illustrated in many of the Puys of central 

 France. There is another series of events, that is to say, the forma- 

 tion of dikes, about which we shall have more to say anon. 



The lava may form a series of little streams over the cone sides, 

 changing their situation according to the point at which the crater is 

 lowest. Here it will cool, forming a buttress of rock on the slopes of 

 the cone. These masses will be covered again by lajnlli, other but- 

 tresses formed in the same manner, and thus the cone built up higher 

 and made stronger. If we see it in section, as in the diagram, it will 

 present a stratification of alternate beds of rock and cinders. This, 

 however, is misleading. The lava-streams do not form a continuous 

 sheet surrounding the cone see diagram, where they are seen cut 

 through in transverse section. When a mountain of some height has 

 been formed, it then becomes liable to fractui'ing, and the formation 

 of so-called volcanic dikes. Mr. Mallet, in a communication to the 

 Geological Society,* thoroughly explained this condition of things. 

 As we have seen, the cone may form an embankment around the 

 column of lava occupying the chimney and crater, consequently there 

 is an enormous pressure put upon the supporting wall of loose mate- 

 rial. Let us begin by taking the pressure of a column of water 

 thirty-two feet high, then let us say another four thousand feet, 

 roughly the altitude of Vesuvius, and compare that with a column of 

 molten lava, whose specific gravity is two or three times that of water. 

 This would be an interesting calculation: given the sj^ecific gravity of 



* " Proc. Geol. Soc," London, vol. xxxii, part iv, p. 4*78. 



