HISTORY OF MADAGASCAR. 
25 
outlet by a covered passage, as, at a considerable distance 
from the well, we heard a rumbling noise under ground, 
resembling that which would be produced by the bubbling 
of gas through water. The boiling appearance arose from 
the discharge of large quantities of carbonic acid gas, which 
constantly escaped from the water. This gas, from its 
superior density to common air, would certainly, at least in 
a quiet day, lie in the cavity of the well, and produce 
fainting in those who entered the water. I showed the 
effects of carbonic-acid gas upon a grasshopper, which 
enabled some of the natives who accompanied us to 
understand the cause of those effects which they had 
described. 
£< At a distance of about a mile and a half, \ve visited 
another spring, or rather an assemblage of springs, rising 
off a bed of limestone. These springs also emitted great 
quantities of carbonic acid gas. One spring, in particular, 
discharged it in such abundance as to produce a noise 
which might be heard at the distance of several yards. The 
strata in this locality appeared to be limestone; and in 
some places there were extensive caves, from the inside of 
the roofs of which beautiful stalactitic formations were sus¬ 
pended. What appeared to us remarkable was the great 
quantities of salt which was found in a state of effloresence 
on these rocks, and also on the damp ground in the 
neighbourhood. The natives of this part of the country 
also drive their cattle to the place, to lick the salt. Here, 
also, instead of planting rice, or grain, or roots, for food, 
as is usual in other places, the natives plant a kind of 
flag, of rapid growth, which imbibes a large portion of the 
saline properties of the soil on which it grows. This rush 
they cut several times in the year, burn it, and from the 
ashes extract a salt, which they pack up in baskets of rush 
