192 PHYSIOGRAPHY. [chap. 



dense showers of such dust have been known tJ darken the 

 sky for miles around the volcano, and have been wafted 

 by winds for even hundreds of miles. It is an interesting 

 fact, shown by the examination of the sea-bottom by the 

 Challenger, that volcanic detritus is almost universally 

 distributed over the floor of the deep sea. 



When the steam, which is abundant in most eruptions, 

 condenses in torrents of rain, the volcanic dust is frequently 

 worked up into a hot mud which rolls down the hill in a 

 sluggish stream, burying everything before it. Herculaneum 

 was sealed up by a crust of volcanic mud discharged from 

 Vesuvius ; while Pompeii was overwhelmed by a vast 

 accumulation of dust and ashes at the same eruption. 



The partially-molten rock called lava rises up in the 

 volcanic pipe, and may eventually run over the lip of the 

 crater, or force its way through cracks in the hill, forming 

 red-hot streams which generally present a consistence some- 

 thing like that of treacle. These lava-torrents are often of 

 great magnitude ; thus, it was estimated that in the famous 

 eruption of Skaptdr Jokul, in Iceland, in 1783, the mass of 

 lava brought up from subterranean regions was equal to the 

 bulk of Mont Blanc. The lava rapidly cools on the surface, 

 though long retaining its heat beneath the protecting crust ; 

 and, ultimately, the entire mass solidifies, forming a hard 

 rock, more or less like a slag from an iron-furnace. In 

 different specimens, however, the lava exhibits great varia- 

 tions ; some being dark-coloured and comparatively heavy, 

 while others are lighter in colour and much less dense ; in 

 some cases the rock is compact, while in others it is spongy 

 or cindery, when it is said to be scoriaceous. The little 

 cavities, or vesicles, in this scoria,^ or cellular lava, are 

 formed by the disengagement of bubbles of gas or vapour, 

 ' Sco!-ia, volcanic cinder, from Lat. scoria, "dross." 



