SPOUTING MUD. 213 



and fifty or three hundred feet above the village of Tur- 

 baco ; but as it was everywhere covered with vegetation, 

 it was not possible to distinguish the nature of the rocks 

 that reposed on the shelly calcareous soil. 



In the centre of a vast plain were eighteen or twenty 

 small cones, in height not above twenty-five feet. These 

 cones were formed of a blackish gray clay, and had an 

 opening at their summits filled with water. On ap- 

 proaching these small craters, a hollow but very distinct 

 sound was heard at intervals, fifteen or eighteen seconds 

 previous to the disengagement of a great quantity of air. 

 The force with which this air rose above the surface of 

 the water led them to suppose, that it underwent a great 

 pressure in the bowels of the earth. Humboldt generally 

 reckoned five explosions in two minutes ; and this phe- 

 nomenon was often attended with a muddy ejection. The 

 Indians assured him, that the forms of the cones 

 suffered no visible change in a great number of years ; 

 but the ascending force of the gas, and the frequency of 

 the explosions, appeared to vary according to the seasons. 

 He found by analyses made by means both of nitrous 

 gas and of phosphorus, that the disengaged air scarcely 

 contained a thousandth part of oxygen. It was azotic 

 gas, much more pure than that which is generally pre- 

 pared in laboratories. 



