1 84 VOLCANIC MUD. 



hood of volcanoes, and in the deep-sea clays far from land. It is 

 more frequently found in the Pacific than in the Atlantic. The pumice 

 is sometimes coated with peroxide of manganese, and may be white, 

 grey, green, or black, as it is felspathic or augitic. It contains crystals 

 of sanidine, augite, hornblende, olivine, quartz, leucite, magnetite, and 

 titaniferous iron. Magnetic iron is found in all the masses. Although 

 a good deal of the pumice may be derived from volcanoes which girdle 

 the Pacific, yet no inconsiderable quantity is derived from land, being 

 washed down from the mountains by the rain, and floated to sea by 

 the rivers. Thus, in Iceland, a ferry is said to have been blocked 

 for days by floating pumice. Quantities of pumice float on the 

 Amazon, brought from the region of its head waters. The river 

 Chile, in Peru, has cut gorges 500 feet deep through pumice, and 

 carries the fragments to sea. 



Mud Streams. Mud streams frequently descend from those vol- 

 canoes which throw out fine ashes. This may be due to different 

 causes. The vast quantity of steam thrown out becomes condensed 

 into rain, and this falling on the mountain, washes down the ashes in 

 torrents of hot mud. Such streams are well known to have descended 

 from Vesuvius in the great eruption of 79, and to have overwhelmed 

 Pompeii and Herculaneum. This substance, however, has now be- 

 come hardened into a compact tuff by the development of zeolites and 

 other minerals in its substance, in precisely the same manner as 

 Bunsen found that basalt, ground to a powder and left in water, con- 

 solidated, when the water evaporated, into a mass so hard as only to 

 bo broken with the hammer. Humboldt has recorded how the 

 volcanoes of Ecuador have discharged torrents of mud so as to fill up 

 valleys ; and it is well known that the cone of Cotopaxi has repeatedly 

 melted the great glaciers upon it, which descend to about 14,000 feet, 

 and the water thus liberated has carried down the ash. The eruption 

 of Imbambaru in 1691 poured out not only mud, but a considerable 

 quantity of fishes, which would indicate that the crater of the volcano 

 had become a lake, in which a species of Pimelodus had lived. In the 

 geological formations, as will be subsequently seen, examples of forests 

 buried in ashes, and vegetation overwhelmed by mud streams, are found 

 among the primary rocks of Arran, and the tertiary rocks of Mull. 



Bombs. As the amount of steam becomes reduced, and the rock 

 is less permeated by it, the explosive force in the volcanic throat is 

 diminished ; and the rock-material, though still blown out into cellular 

 structure by the expansion of the steam, is ejected in much larger 

 fragments, which are termed lapilli and scorice. At length the supply 

 of explosive steam near to the surface becomes exhausted, and the 

 fluid rock ceases to be shattered by its expansion. But from time to 

 time fresh supplies of condensed vapour rise through the molten rock 

 as it ascends, and catch up masses of lava, which are often rotated as 

 they rise in the air, and become fashioned into the often rounded and 

 sometimes hollow masses termed " volcanic bombs." These, however, 

 are rarely thrown far, and usually occur near to the cone. But 

 grander paroxysmal outbursts of steam have lifted large masses of 



