


224 DYNAMICAL GEOLOGY. ~~ [Boox Ill. — 
Here and there inside this crust the lava subsides, leaving cavernous 
spaces and tunnels into which, when the whole is cold, one may 
creep, and which are sometimes festooned and hung with stalactites — 
of lava. 
Size of lava-streams.—In some cases lava escaping from 
craters or fissures comes to rest before reaching the base of the slopes, 
like the obsidian current which has congealed on the side of the little 
volcanic island of Volcano. In other instances the molten rock not only 
reaches the plains but flows for many miles away from the point of 
eruption. The most stupendous outpouring of lava on record was 
that which took place from Skaptar Joékull in Iceland in the year 
1783. Successive streams issued from the volcano, flooding the 
country far and wide, filling up river-gorges which were sometimes 
600 feet deep and 200 feet broad, and advancing into the alluvial 
plains in lakes of molten rock 12 to 15 miles wide and 100 feet deep. 
Two currents of lava which flowed in nearly opposite directions 
extended for 45 and 50 miles respectively, their usual thickness being 
100 feet, but in narrow defiles reaching sometimes to 600. Bischof 
estimated that the total amount of lava poured forth during this 
single eruption “surpassed in magnitude the bulk of Mont Blane.” ?} 
Varying liquidity of Lava—aAll lava is at the time of its 
expulsion in a molten condition, that is, consists of a glassy magma 
in which, by reason of the high temperature, most or all of the 
mineral constituents exist dissolved. Considerable differences, how- 
ever, have been observed in the degree of liquidity. Humboldt and 
Scrope long ago called attention to the thick, short lumpy forms 
presented by trachytic rocks, which are lighter and more siliceous, 
and to the thin, widely extended sheets assumed by basalts, which 
are heavy and contain much iron and basic silicates. It may be 
inferred that as a rule the basalts or more basic lavas have been 
more liquid than the trachytes or more siliceous lavas, The cause’ 
of this difference has been variously explained. It may depend © 
partly upon chemical composition, the siliceous being naturally less 
fusible than the basic rocks. 
But as great differences of fluidity are observable even among 
Javas having nearly the same composition, there would seem to be 
some further cause for the diversity. Reyer has ingeniously 
maintained that we must look to original differences in the extent to 
which the subterranean igneous magma which supplied the lava has 
been saturated with vapours and gases. Molten rock highly 
impregnated gives rise, he holds, to fragmentary discharges, while 
when feebly impregnated it flows out tranquilly.? On the other 
hand Captain C. IX. Dutton, who has recently studied the volcanic 
| apes of Western America, suggests that the different degrees of 
iquidity may depend, not only on chemical differences, but on 
' Lyell, Principles, ii. p. 49. 
* Serope, “ Considerations on Voleanoes ” (1825), p. 93. 
* Beitrag zur Physik der Eruptionen, p. 77. w 
