182 ERUPTION OF ASHES FROM COSEGUINA. 



no lava appears to have been produced, but enormous quantities of 

 dust and ashes, which sometimes ran down the mountain in torrents 

 of mud ; while at other times Vesuvius has ejected ashes and lava. 



The differences in these conditions are attributable to the supply of 

 water. Professor Dana has inferred from the fact that borings on a 

 sea-shore will always yield fresh water, that the water is more likely 

 to be fresh than salt ; and that the rains and melting snows, absorbed 

 by the cavernous rock of which a volcano consists, may contribute 

 materially to supplying the explosive force to the fires below. But, 

 on the other hand, many of the springs in Italy contain a perceptible 

 amount of salt; and since the volcanic fires rise from a depth far 

 below the sea level, the pressure of the sea must exert itself in forc- 

 ing water into the rocks. It may be doubtful, perhaps, how far the 

 substances dissolved in sea-water contribute to volcanic phenomena, 

 but we may remember that though the salts consist chiefly of chloride 

 of sodium and magnesia, lime and potash, chiefly in the form of 

 chlorides and sulphateSj Dr. Forschammer detected minute quantities 

 of a large number of elements^ such as from time to time are met 

 with in volcanic and other igneous rocks; 



Steam. If we suppose water ready to take the form of steam, to 

 accumulate beneath the surface until the pressure becomes so great 

 as to burst a way through the rocks, as they become flexured, frac- 

 tured, and heated, and then to expand, it becomes intelligible that the 

 steam will be discharged with such force as to rise to a great height 

 in the air, before it is chilled so as to condense in clouds. The force 

 of the steam accounts for the abrasion and trituration of rock-fragments 

 from the side of the eruptive throat, which, as we have already seen, 

 are sometimes brought to the surface. Eut after a time, especially in 

 the volcanoes of the south of Italy, dust is thrown up with the steam. 

 This dust is not merely grated down from the rocks at the sides of 

 the fissure, but is at first of such indescribable fineness as to remain 

 long suspended in the air. The heated rock beneath the surface is in 

 fact so charged with water that, when it comes towards the surface, 

 the water expands into steam, blowing out the films like soap bubbles, 

 so that they cool and contract ; and becoming broken, fall into the 

 finest powder. 



Eruptions of Ashes. An excellent example of a dust eruption took 

 place in the volcano of Coseguina in 1835. This cone is situate on 

 the Bay of Fonseca, in Nicaragua, and is about 500 feet above the sea. 

 Slight noises were heard, and smoke was seen on the i gth of January, 

 and on the 2oth a cloud was thrown up, which, seen from San Antonio, 

 48 miles south, looked like an immense plume of feathers ; at first 

 white, then grey, yellow, and finally crimson, and expanding rapidly 

 in every direction ; columns of fire shot up, and there were severe 

 earthquakes. On the 22d the sun shone brightly, but in the direction 

 of the cloud there was intense darkness. A fine white ash began to 

 fall, and in half an hour the day, at San Antonio, became darker 

 than the darkest night, so that people could touch without seeing 

 each other ; the cattle came in from the surrounding country, and the 



