304: Dr. Johnston-Lavis — Vesuvius and Monte Somma. 



Lava we have seen to be rather the result of chronic activity ; 

 which is obviously due to two causes. First, as there is a continual 

 dribbling forth tVom time to time of tiuid rock, that jjortion occupy- 

 ing the chimney is gradually changed, so as to leave little time for 

 hydration of the magma. Secondly, whatever aqueous matter was 

 absorbed, would be rapidly got rid of as bubbles which rise to the 

 surface of the fluid column and there escape as the " smoke " of the 

 volcano. 



In chronic activity the lava column may be kept a long time, 

 independent of constant loss, at a high temperature, from the heat 

 derived from below ; so that crystallization can have its full play, 

 producing highly crystalline rocks. Thus we see at Vesuvius that 

 the small streams that issue after a long period of gentle activity are 

 exceedingljr rich in large leucites and pyroxenes, whereas long 

 streams after an inactive state are fine-grained. With the coarse 

 crystalline lavas all the water has been boiled off so that no scoria 

 forms, as in that of 1858, ^ whereas in the fine-grained vapour-bearing 

 ones, as 1631 and 1871i, there is an abundance of scoria formed on 

 their surface. 



Dyke-forming material may be regarded as a lava suddenly cooled 

 at its surfaces as a vitreous salband. The central portion on the 

 contrary cools slowly imder comparatively favourable conditions, com- 

 pared with the simultaneous outflowing of lava in another direction, 

 while, though the latter may be fine-grained, the former may have 

 a very coarse crystalline structure. 



All these variations in eruptive phenomena do not exclude those 

 other conditions that exist in the fluid (?) interior of the earth, or at 

 least in the reservoirs from which the primary matter must be 

 derived. The upheaval of mountain chains, a certain class of wide- 

 spread earthquakes, the apparent relation of eruptive periods to the 

 lunar tides, and barometric pressure, may all be independent of the 

 secondary or modifying causes of eruptive activity. 



If, as is generally the case, the volcano be situated near the sea, 

 there is no reason why the old theory of percolation of sea-water 

 may not in part be true as a cause of eruption. We, however, must 

 not look upon water as the primary determining agent, for if the 

 seismic forces are capable of upheaving the great mountain ranges 

 of the globe, surely they can fissure the crust of the earth and squeeze 

 out a little fluid rock. Such escape might take place quite tranquilly, 

 were the igneous magma not brought into contact with water-bearing 

 strata, and probably sometimes does so, if we may judge from those 

 extensive sheets of basalt in Idaho, U.S.A., without the sign of a 

 cinder cone. 



That sea-water really reaches the lava, and causes its more violent 

 exit, seems certainly supported by the fact that the upholders of 

 that theory have made so much use of, namely, the quantity of 

 H CI, with chlorides and sulphates of the alkalies so abundant in 

 volcanic vapour, and around fumaroles. Most valuable evidence 

 could be obtained by the study of the saline crusts and exhalations, 

 if any exist, that occur at the few inland volcanoes that are at 

 1 Judd, Volcanoes, 1881, p. 211. 



