126 



Prof. J. Prestwich. 



discharge of steam and lava could not fail to bear some proportion one 

 to the other. 



Although the phenomena accompanying volcanic eruptions are so 

 constantly recorded, those which bear in particular on this question 

 are generally so mixed up with the other details, that it is not 

 always possible to determine their relative bearing and sequence. 

 There are, however, an ample number of cases to show that the dis- 

 charge of lava is not in proportion to the discharge of steam, nor is 

 the discharge of steam always in accordance with the escape of 

 lava, which they should be if the hypothesis were correct. These con- 

 ditions would seem on the contrary to be perfectly independent one of 

 the other. It is of course conceivable that lava of an extreme fluidity 

 and offering less resistance to the escape of the elastic vapours, might 

 be ejected in lesser quantity than a more viscid lava, which presented 

 more resistance, or that paroxysmal explosions may disperse the lava 

 in aerial discharges, and reduce the importance of the quieter outflow ; 

 but these occasional occurrences would not affect the more general 

 results. There are too many great eruptions that have been attended 

 with a small discharge of lava, and too many of the largest lava 

 streams have been erupted quietly and with a very small exhibition of 

 explosive violence, to allow of much doubt on the subject. Sometimes, 

 when the discharge of lava has been at its maximum, the explosive 

 violence has been at its minimum, and, on the other hand, violent 

 detonations have been attended with small overflows of lava. 



According to Daubeny,* there is no recorded lava-flow accompanying 

 the eruptions of Vesuvius prior to the eruption of a.d. 1036. This, 

 however, may be the mere absence of record. Still, it would seem to 

 point to the prevalence of paroxysmal eruptions like that of the great 

 eruption of 79 B.C. 



Mr. Scrope divides volcanic outbreaks into periods of moderate 

 activity and of paroxysmal violence, f and he himself remarks that 

 " the volume of lava poured out by an eruption does not preserve any 

 constant proportion to the force or continuance of its explosions." 

 He instances EtnaJ as an example of almost continual moderate 

 activity with occasionally more or less paroxysmal outbursts. The 

 volcano of the Island of Bourbon offers another example of the same 

 kind. He further points out§ that " in all cases where lava is emitted 

 its protrusion marks the crisis of the eruption, which usually attains 

 the maximum of its violence a day or two after its commencement. 

 The stoppage of the lava in the same manner indicates the termina- 

 tion of the crisis, but not of the eruption, for the gaseous explosions 



* " Description of Volcanos," 2nd Edit., 1848, p. 225. 

 t " Volcanos," 2nd Edit., p. 16—19. 

 X Ibid., p. 24. 

 § Op. tit., p. 23. 



