298 DYNAMICAL GEOLOGY. — [Boox IIT. 
which have reached the surface, is a fact of no little significance in 
the theoretical consideration of hypogene action. ‘ 
Inclination of lava-flows.—lIt was at one time supposed that 
lava could not consolidate in beds on such steep slopes as those 
of most volcanoes. Hence arose the “elevation-crater theory” 
(described at p. 240), in which the inclined position of lavas round 
a voleanic vent was explained by upheaval after their emission. 
Observations all over the world, however, have now demonstrated that 
lava, with all its characteristic features, can consolidate on slopes of — 
even 35° and 40°. The lava in the Hawai Islands has cooled 
rapidly on slopes of 25°, that from Vesuvius, in 1855, is here and 
there as steep as 30°, while the older lavas in Monte Somma are 
sometimes inclined at 45°. On the east side of Ktna, a cascade of lava, 
which poured in 1689, into the vast hollow of the Cava Grande, has 
an inclination varying from 18° to 48°, with an average thickness 
of 16 feet. On Mauna Loa some lava-flows are said to have 
congealed on slopes of 49°, 60°, and even 80°, though in these cases it 
could only be a layer of rock stiffening and adhering to the surface 
of the declivity. Even when it consolidates on a steep slope, a 
stream of lava forms a sheet with parallel upper and under surfaces, 
a general uniformity of thickness, and often greater evenness of 
surface than where the angle of descent is low. ‘The thickness 
varies indefinitely ; many basalts which have been poured out in a - 
remarkably liquid condition have solidified in beds not more than 10 — 
or 12 feet thick. On the other hand more pasty lavas, especially 
where they have flowed into narrow valleys, may be piled up into 
solid masses to a thickness of several hundred feet. ; 
Structure of a lava-stream.—Some lava-streams are nearly 
homogeneous throughout. In general, however, they each show 
three component layers. At the bottom lies a rough, slaggy mass, 
produced by the rapid cooling of the lava, and the breaking up and 
continued onward motion of the scoriform layer. The central and 
main portion of the stream consists of solid lava, often, however, 
with a more or Jess carious and Vesicular texture. The upper part, 
as we have seen, may be a mass of rough broken-up slabs, scorie, or 
clinkers. The proportions borne by these respective layers to each 
other vary continually. Some of the more fluid ropy lavas of 
Vesuvius have an inconstant and thin slagey crust; others may be> 
said to consist of little else than scoriz from top to bottom. Through- 
out the whole mass of a lava-current, but more especially along its 
upper surface, the absorbed vapours expand as the pressure diminishes, 
aud pushing the molten rock aside, segregate into small bubbles 
or irregular cavities. ence, when the lava solidifies, these steam- 
holes are seen to be sometimes so abundant that a detached portion 
of the rock containing them will float in water (pumice). They are 
often elongated in the direction of the motion of the lava-stream (Fig. 
44). Sometimes, indeed, where the cells are numerous, this elonga- 
tion of them in one direction gives a fissile structure to the rock. 

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