The Eastern Congo 



Now, let us suppose that one of these earthquakes or 

 earth contractions formed a fissure in the bed of the ocean 

 or a lake (either a surface or subterranean one) and that the 

 ramifications of this fissure connected up with a lake of red- 

 hot lava. The result would be an explosion together with the 

 formation of vast quantities of steam. We have now an 

 embryo of an eruption and after this it needs very httle 

 effort of the imagination to follow the combined lava, water 

 and steam under terrific pressure, in its efforts to escape along 

 the line of least resistance. 



Thus we have the phenomena of volcanic eruptions. 



Perhaps not in all instances caused in exactly this manner, 



for it has been suggested that eruptions may be brought 



about by mere percolation of water on to the red-hot 



lava. 



It has been calculated, moreover, on the evidence of the 



fact that the average increase of temperature from the surface 



of the earth in a downward direction is approximately i°F. 



for every sixty feet, that the burning lava thrown out by an 



eruption comes up from a depth below ground of from twenty 



to thirty miles. 



Before going on to describe the Virunga volcanoes in 



particular it would be as well to state the fact that volcanoes 



are put into three classes, viz., active, dormant and extinct. 



An active volcano, as we all know, still continues at intervals 



to break into eruption. A dormant one, however, is one which 



after being quiescent for a long interval, as if its fires were 



extinct, suddenly breaks forth anew and is therefore the most 



to be feared. The third and last is the extinct volcano, 



which is one not known to have been in eruption since man 



has been upon the earth. 



62 



