97 
This engraving was executed from a sketch made 
by one of our friends, Mr. Lewis de Rieux. 
This young artist, with whom we ascended the 
river Magdalena, was then attending his father, 
who, under the administration of Mr. d’Urquijo, 
was charged with the inspection of the bark trees 
of Santa-F6. 
In the centre of a vast plain, bordered by 
bromelia karatas, are eighteen or twenty small 
cones, in height not above seven or eight me- 
tres. These cones are formed of a blackish gray 
clay, and have an opening at their summits 
filled with water. On approaching these small 
craters, a hollow but very distinct sound is heard 
at intervals, fifteen or eighteen seconds previous 
to the disengagement of a great quantity of air. 
The force with which this air rises above the 
surface of the water may lead us to suppose, that . 
it undergoes a great pressure in the bowels of 
the Earth. I generally reckoned five explosions 
in two minutes : and this phenomenon is 
often attended with a muddy ejection. The 
Indians assured us, that the forms of the cones 
undergo no visible change in a great number of 
years ; but the ascending force of the gas, and 
the frequency of the explosions, appear to vary 
according to the seasons. I found by analyses 
made by means both of nitrous gas and of 
phosphorus, that the disengaged air scarcely 
VOL. xrv. H 
