220 DYNAMICAL GEOLOGY. 

rising into rugged mounds or getting seamed with rents and gashes, — 
at the bottom of which the red-hot glowing lava may be seen 
(Fig. 39). In lavas possessing somewhat greater fluidity, the surface | 
presents froth-like, curving lines, as in the scum of a slowly flowing 
river, or is arranged in curious ropy folds, as the layers have suc- 
cessively flowed over each other and congealed. These, and many 
other fantastic coiled shapes were exhibited by the Vesuvian lava of. 
1858. SBasalts possessing extreme liquidity have flowed for great 
distances with singularly smooth surfaces. A large area which has 
been flooded with lava is perhaps the most hideous and appalling 




























































































































MARTI, SC, 
Fig. 39.—Vinw OF PORTION OF A LAVA-STREAM ON Vesuvius (ABICH). 
scene of desolation anywhere to be found on the surface of the 
globe. 
A lava stream usually spreads out as it descends from its point 
of escape, and moves more slowly. Its sides look like huge embank- 
ments, or like some of the long mounds of “clinkers” in a great 
manufacturing district. The advancing end is often much steeper, 
creeping onward like a great wall or rampart, down the face of 
which the rough blocks of hardened lava are ever rattling (Fig. 40). 
Outflow of Lava,—This appears to be immediately due to the 
expansion of the absorbed vapours and gases in the molten rock. 
Though these vapours may reach the surface and even produce tre- 
mendous explosions without an actual outcome of lava, yet so 
intimately are vapours and lava commingled in the subterranean 
reservoirs, that they commonly rise together, and the explosions of 
the one lead to the outflow of the other. The first point at which 
