30 Eruption of Vesuvius, 1867-8 



a thousandfold more violent by the expansion and explosive force 

 of the imprisoned gases. And v^here more likely that such cracks 

 should occur, than just along those lines where the land loses and 

 the sea gains most. Point to an active volcano, if you can, away 

 from the sea coast. Etna, Vesuvius, Hecla, TenerifFe, Cotopaxi, 

 Torullo, Erebus, all overlook the sea; and the only inland volcano 

 to be found is Mount Demawend in Persia, which is itself on the 

 shore of the greatest lake in the world — the Caspian. 



Volcanic eruptions are almost always preceded by earthquakes, 

 by which beds of rock that overlie and keep down the struggling 

 powers beneath are dislocated and cracked, till at last they give 

 way, and the strain is immediately relieved. It is when this does 

 not happen, when the force below is sufficient to heave up and 

 shake the earth, but not to burst open the crust and give vent to 

 the lava and the gases that the greatest havoc takes place. Such 

 was the earthquake of Lisbon, the shock of which extended even 

 as far as the north of Scotland, or that which but a short while ago 

 desolated Mitylene. 



Lastly, a few words as to the lava itself. Lava is not one 

 mineral, but manj' minerals, being a general name for all mineral 

 matter that issues, or has issued from a volcano in a state of 

 fusion. Jlence one reason of the great variety in its shape and 

 colour, to which also the rapidity or slowness with which it cools 

 has much to say. When it first wells up it is perfectly liquid, of 

 an intense white heat, but a very few yards of flow are sufficient to 

 reduce this temperature considerably. The stream as it grows 

 more cohesive becomes more sluggish and ropy, especially where 

 it comes in contact with the sides or bottom of its channel, to 

 which it has to impart some of its heat. In consequence of this, 

 the surface takes a convex form, the centre of the stream flowing 

 faster than the rest, and at last from exposure to the cold, and 

 probably also to the escape of heated vapour from the interior of 

 the mass, cracks into innumerable fragments. These lie in a deep 

 layer on the surface, or roll to either side of the oonvex current, 

 and form those banks or walls 1 have mentioned, and which, from 

 their extreme regularity we could hardly believe to be natural. 

 The scoriae which lie on the surface are bad conductors of heat, 

 and enable the lava to retain its fluidity for months and even 

 years, while they also make it possible to cross the current, 

 though I cannot say I should have liked to be put to the fiery 

 ordeal. 



One more fact about the lava, another instance in that series 

 of counterbalances that pervades all nature. However bad a 

 neighbour a volcano is in one sense, the lava pays its way by the 

 extraordinary fertility of the soil composed of its pulverized 



