VOLCANOES 3 345 
logged, and settled over the bottom of the Pacific, thus making a 
distinct contribution to the sediment of the ocean floor. 
There have been many eruptions of great violence, but most 
volcanic outbreaks are moderate indeed in comparison with this. 
Still, even the mildest eruption is a most impressive evidence of 
the great power that exists within the earth, and is able to cause 
the ejection of liquid rock from points miles below the surface. 
Effects of Volcanoes.— One of the most momentous 
effects of volcanic action is the destruction of life. 
Such loss is due not merely to the ash and lava, but 
also to the water wave that is often started by the 
eruption, and to the earthquake shocks which result. 
By these the lives of plants, of animals, and of man 
himself are endangered. Sometimes animals and plants 
are entirely exterminated for distances of many miles 
from the centre of eruption, and the earthquake shock 
and water wave often carry the disaster still further. 
By this destruction of life, fossils of animals and plants 
are frequently buried and preserved beneath the ash 
or lava, and thus is maintained a record of some of 
the organisms then living. -In the case of Pompeii and 
Herculaneum, which were buried during the eruption 
of Vesuvius in 79 A.D., cities and works of man have 
been perfectly preserved (Fig. 209). 
Flows of lava extend over the land and sometimes 
pour into the sea, where they are perhaps covered by 
later sedimentary rocks. Then we have lava sheets 
included between the members of a series of sedimen- 
