88 A VOYAGE TO Book VII. 



Gompreiies it by its greater expanfion, and at the fame 

 time rarifies it by its heat ; but the cavities, being too 

 narrow to admit of its proper dilatation, it ftruggles for 

 a vent, and thefe efforts fnake all the contiguous 

 parts till at iafl, were it finds the leaft refiftance, 

 it forces itfelf a palTage, which fonietimes clofes 

 again by the tremulous motion it occafions, and at 

 others continues open ; as may be feen in different 

 parts of all thefe countries. When on account of 

 the refinance being equal, it finds a paflage in feve- 

 ral parts, the chafms or fifiures are generally fraalier, 

 fo that rarely any veftiges remain after the concuf- 

 fion. At other times when the fubterranean cavities 

 are fo large as to form fubterranean caverns, they not 

 only rend the earth, and at every Ihock leave it full 

 of difruptions but alfo caufe it to fink into fpa- 

 cious hollows ; as I particularly obferved near the 

 town of Guaranda, a place in the jurifdidion of 

 Chimbo, in the province of Qiiito ; v/here in the 

 year 1 744 all the ground on one fide of the chafm 

 funk near a yard, the other fide rifing in the fame 

 proportion, though with fome inequafities on both 

 fides. 



The loud fubterraneous noifes preceding earth- 

 quakes, and which imitate thunder at a diftance, 

 feem to correfpond with the abovementioned caufe and 

 formation of earthquakes, as they can only proceed 

 from the rarefadtion of the air, on the ignition of the 

 explofive fubfiiances, v/hich being impetuoufly propa- 

 gated thro' all the caverns of the earth ; propelling, 

 and at the fame time dilating what is contained in 

 them, till all the cavities being pervaded, and no vent 

 found, the efforts for a further dilatation, begin, and 

 form the concufuon with which it terminates. 



It mufi: be obferved that at the time when the 

 air, which had been confined within the earth, burfi:s 

 through it, neither the light nor fire emitted from 

 the chafms are feen. The reafon is, that this light 



an4' 



