260 
"VARIETY OF STRUCTURE. 
Ottomite Indians, and the Bridge of the Mother of God, 
bv the Mexican Spaniards. It is difficult to decide respect- 
ing the origin ot these channels, "which sometimes serve 
as beds for subterranean rivers. Are these pierced rocks 
hollowed out by the impulse of a current ? or should we 
rather admit that one of the openings of the cavern is 
owing to a falling down of the earth subsequent to its 
original formation; to a change in the external form ot 
the mountain, for instance, to a new valley opened on its 
Hank ? A third form of caverns, and the most common 
of the whole, exhibits a succession of cavities, placed nearly 
on the same level, running in the same direction, and com 
municating with each other by passages of greater or less 
breadth. 
To these differences of general form are added other 
circumstances not less remarkable. It often happens, that 
grottoes of little space have extremely wide openings; 
whilst we have to creep under very low vaults, in order to 
penetrate into the deepest and most spacious caverns. The 
passages which unite partial grottoes, are generally hori- 
zontal. I have seen some, however, which resemble tunnels 
or wells, and which may be attributed to the escape of some 
elastic fluid through a mass before being hardened. When 
rivers issue from grottoes, they form only a single, horizontal, 
continuous channel, the dilatations of which are almost 
imperceptible; as in the Cueva del Guaclmro we have just 
described, and the cavern of San Felipe, near Tehuilotepec 
in the western Cordilleras of Mexico. The sudden disappear- 
ance* of the river, which took its rise from this last cavern, 
has impoverished a district in which farmers and miners 
equally require water for refreshing the soil and for working 
hydraulic machinery. 
Considering the variety of structure exhibited by grot- 
toes in both hemispheres, we cannot but refer their for- 
mation to causes totally different. When we speak of the 
origin of caverns we must choose between two systems of 
natural philosophy : one of these systems attributes every 
thing to instantaneous and violent commotions (for example, 
to the elastic force of vapours, and to the heavings occasioned 
* In the night of the 16th April, 1802. 
