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VOLCANIC CRATERS AND EXPLOSIONS. 
A further stage of the same process occurred in Mont Pelee, in Marti¬ 
nique, in the great eruption of 1902 , in which the city of St. Pierre was 
destroyed. A dome was first formed in the bottom of the old crater, but 
did not nearly fill it. The material of which it was composed contained 
large volumes of dissolved gases, and broke down as described above in 
the case of the Soufriere of St. Vincent. The incandescent mixture passed 
out through a Y-shaped notch in the side of the crater, rolled down the 
mountain-side as an avalanche, and destroyed St. Pierre. 
As the eruption became less violent a plug formed in the volcanic 
chimney as above described, and being pressed forward from below, 
forced its way through the dome, and formed the famous spine.f It rose 
to a height of about 800 feet above the top of the mountain, and it is 
believed that material to the extent of above 1000 feet was actually pushed 
out ; but it was crumbling away all the time, and when I ascended the 
mountain twice, about five years later, only a rounded stump was visible, 
with a ring of very active fumaroles marking its junction with the dome, 
and with the talus of fallen debris. 
In the old world we are accustomed to regard volcanoes as mainly 
responsible for the discharge of material and the building up of cones and 
deposits of lava and tuff, but in the new world, and especially in the 
Pacific, the geologists are accustomed equally naturally to invoke the 
agency of subsidence as the cause of any otherwise inexplicable hollow. 
During a recent visit there I took the opportunity of examining as many as 
possible of such real or supposed subsidences, and I select for our present 
discussion some of the most characteristic. 
In an ordinary explosion crater, towards the waning of the eruption, 
there is often deposited a large bulk of material containing clay and various 
sulphur compounds. The gases which rise from the vent during the 
solfatara stages often also contain sulphur compounds. These, by 
contact with the air, tend to become oxydized into sulphuric acid, which, 
combining with the alumina of the clay, forms alum. Water percolating 
through this dissolves and removes the alum, and subsidence of the surface 
takes place. Such undoubted subsidences vary in area from the size of an 
ordinary sitting-room to several acres. Many such subsidences are found 
in the volcanic district of New Zealand, as at Waiotapu,*f Koto Kawa, and 
Wairakei, j and of the reality of the cause in their cases I feel no doubt, 
but it is more difficult to accept it when a lake several miles in diameter 
is under consideration, and where no remains of alum beds are visible. 
The great volcano of Kilauea in Hawaii, or Owyhee as it used to be 
called, is a good example of the type of efflusive volcanoes as they occur in 
the Pacific, and its crater is often quoted as an example of a subsidence. 
The lava discharged from it is so fluid that the slope of the outside of the 
cone is only 5 ° or 6°, and in places I believe as low as 2 °, so that the crater 
appears to be more a hole in a plain than in the top of a mountain, while 
t Slide shown. 
