La vis — On the Structure of Rocks. 135 



(c) Amount of enclosed volatile matter. 



(d) Amount of pre- eruptive crystallization. 



(e) Rapidity of ejection. 

 (/) Height of projection. 

 (g) Temperature of the air. 



The ejection will take place at first with great rapidity, but 

 will diminish as the tension in the whole unescaped mass is 

 relieved. But beyond this the upper portion of the injected 

 igneous magma column will be more exposed to aquiferous strata 

 than that farther removed from the earth's surface; and con- 

 sequently the expansion, and the results dependent upon it, will 

 be most marked in the portion of the magma near the surface, 

 and also it is probable that that part richer in water will be 

 lighter, and rise to the top of the column. This part having 

 escaped, those portions that follow it will be hotter from di- 

 minished loss of heat, from the less amount of diffused water 

 it has raised to its own temperature, and also from the less water 

 to be converted into steam : the latter will escape more slowly, 

 and will reach a less height, all circumstances favourable to the 

 slower cooling and less vesicularization of the magma. The con- 

 sequence is, that we must expect more crystalline and denser ejecta- 

 menta generally in larger fragments, which I have called pumiceous 

 scoria. Should the eruption not terminate at this point, the con- 

 ditions favourable to slow cooling, more complete crystallization, 

 and continuity of mass, may proceed to such a point that the 

 igneous magma may pour forth from the vent, forming lava 

 streams of vast extent, so that years may be occupied in cooling, 

 or the magma may be kept simmering in the volcanic chimney, 

 presenting the characters of strombolian action. Monte Nuovo is 

 a good illustration of the former case, although the lava hardly 

 reached the point of flowing out as a continuous mass. The pro- 

 gress of events, as above described, is fully borne out by the 

 investigations of the physical structure and composition of rocks, 

 whose mode of formation we can judge of by historical accounts, 

 by collateral facts, and by analogy. I first discovered that an 

 eruption of explosive type produced a deposit of pumiceous nature, 

 divisible into three sections, at Vesuvius, 1 and I have been able to 



1 " Geology of Mt. Somma and Vesuvius," &c, Q. J. Geol. Soc, Jan. 1884. 



