145 



netrate into the deepest and most spacious ca 

 verns. The passages, which unite partial grot- 

 toes, are generally horizontal. I have seen 

 some, however, which resemble funnels or wells, 

 and which may be attributed to the disengage- 

 ment of some elastic fluid through a mass not 

 yet hardened. When rivers issue from grottoes, 

 they form only a single, horizontal, continued 

 channel, the dilatations of which are almost im- 

 perceptible ; such are the Cueva del Guacharo, 

 which we have just described, and the cavern of 

 St. Philip, near Tehuilotepec, in the western 

 Cordilleras of Mexico. The sudden disappear- 

 ance 5 ^ of the river, which took it's rise from this 

 last cavern, has caused the impoverishment of a 

 district, the farmers and miners of which have 

 equally need of water, to refresh the fields, and 

 to move their hydraulic machines. 



On considering this variety of structure ex- 

 hibited by the grottoes in both hemispheres, we 

 are compelled to refer their formation to causes 

 totally different. When we speak of the origin 

 of caverns, we must choose between two sys- 

 tems of natural philosophy, one of which attri- 

 butes every thing to instantaneous and violent 

 commotions ; for instance, to the elastic force 

 of vapours, and to the heavings occasioned by 

 volcanoes ; while the other has recourse to small 



* In the night of the 16th of April, 18C2. 

 VOL. III. h 



