608 
DISAPPEARI^TG OF RtVEUS. 
the Aure, is the nature of the soil through which they pass. M. 
Gnettard has observed, that it is in general porous, and composed of 
a thick sand, the grains of which are not well compacted together : 
it sinks suddenly down by its own weight in some places, and there 
forms great holes ; and when the water overflows the meadows, it 
frequently makes many cavities in several parts of them. If we 
therefore suppose inequalities in the channels of these rivers, and 
that there are certain places in which the water stagnates longer than 
in others, it must there dilute the ground, if we may use that expres- 
sion, and having carried away the parts which united the grains of 
sand together, those grains will become afterwards no other than a 
kind of sieve, through which the w'aters w'ill filtrate themselves, pro- 
vided nevertheless that they find passage under ground, through 
which they may run. This conjecture appears to be so well founded, 
that each of these three rivers loses itself nearly in the same manner, 
that is, through cavities which the people of the country call Betoirs, 
and which swallow up more or less, according to their largeness. 
M. Gnettard, who has carefully examined them, remarks, that 
these betoirs are holes in the form of a tunnel, whose diameter and 
aperture are at least two feet, and sometimes they exceed four; and 
whose depth varies in like manner from one and two feet, to five, six, 
and even twenty. The water generally gets into these cavities when 
the river is not very high, making a gurgling noise, and turning round 
in an eddy. A proof that waters are there filtered and absorbed 
among the grains of- the sharp diluted sand, is, that frequently in a 
betoir two or three feet deep, and through which a great deal of 
water is lost, one cannot thrust a stick further than the surface of its 
bottom. Wherefore, as these betoirs so frequently occur in the bed 
and hanks of the Rille, the Ithone, and the Aure, it is not surprising 
that these rivers should be thus lost. The Rille, during the summer 
season, loses almost all its water in the space of tw'o short leagues ; 
the Ithone does very near the same. But M. Guettard observes some- 
thing curious concerning this river, to wit, that formerly it was not 
lost, but kept its course without any interruption, as appears by the 
history of the country ; very likely the mud, which had been collected 
together in several parts of its channel, might have occasioned the 
waters remaining in others, and thereby caused many betoirs. This 
is the more probable, as the mud having been collected together in 
the bed of the river Aure, it appears that, in consequence thereof, the 
cavities were greatly increased, which makes it lose itself much sooner 
than formerly ; however, it has been resolved to cleanse its, channel, 
to remove this inconvenience ; besides, possibly an earthquake happen- 
ing in the country might have caused several subterraneous canals, 
through which the water of the Ithone has forced its way. In effect, 
it appears that a soil’s being porous is not sufficient to cause the 
loss of the river, for if it were, then it would occasion many fens 
round about, nor would it renew its course after having disappeared 
a certain time. 
M. Guettard supposes that there are in these parts subterraneous 
cavities, through which the waters may flow ; and in consequence of 
this he reports a number of facts, all tending to prove that there 
