» 




226 DYNAMICAL GEOLOGY. © — _[Boox I. 5 
lava dome with ejected scoriz, and had opened a number of — 
crateriform mouths on its summit.* 9 
There can be no doubt, as above remarked, that the condition of | 
liquidity of the laya has in some measure determined the form 
of the eruptions. In one ease there are quiet out-wellings of the 
more liquid lavas, as at Hawaii; in another there are explosive 
discharges and cinder cones accompanying the more viscid lavas, 
as at most modern voleanoes, ‘The former has been the condition 

























































































































“ \Y \ 



















































































































































































































































































































































x 7 
4 Yao * ‘3 
MEL me 
MARTINS 
> i Pe as = ee Ze wy 
Si ieee ee ol 
* 
Fie, 43.—Lava CoLuMN (f1GHT YEET HiGH), VEsUvIUS (ABICH). 
favourable to the most colossal outpourings of molten rock, as we 
see in the basalt plateaux of Britain, Faroe, Greenland, Idaho, 
and Oregon, the Ghauts, Abyssinia, &c. This subject is again 
referred to at p. 256. 
Crystallization of Lava.—Pouring forth with a liquidity like 
that of molten iron, lava speedily assumes a more viscous condition and 
a slower motion. Obsidian and other vitreous rocks have consolidated 
as glass, Yet that they are not always extremely fluid is indicated by 
the arrest of the obsidian stream half way down the steep northern 
slope of Volcano, Even in such perfect natural glass as obsidian, 
' Fouqué, op. cit. p. xv. e 
