594 ulloa's voyage to south America* 



the mountain may take fire, the convulfion of the earth is feldom or never felt a 

 , fecond time. The reafon of which is, that . the fudden reiteration of this accident 

 greatly augments the volume of the air by rarefacrion ; and as it finds an eafy p7'Svj.ge 

 without labouring in the bowels of the earth for a vent, no other conculTion is pro- 

 duced than what muft follow from the eruption of a great quantity of air through an 

 aperture too narrow for its volume. 



The formation of volcanoes is now well known ; and that they owe their origin to 

 yij^^^^^,,fuIphureous, nitrous, and other combuftible fubftances in the bowels of the earth ; for 

 thefe being intermixed, and, as it were, turned into a kind of pafte by the fubterraneous 

 waters, ferment to a certain degree, when they take fire ; and by dilating the contiguous' 

 wind or air, and alfo that within their pores, fo that its volume is prodigioufly increafed 

 beyond what it was before the inflammation, it produces thfe fame efteQ: as gunpowder' 

 when fired in the narrow fpace of a mine, but with this difference, that powder on being 

 fired immediately difappears, whereas the volcano being once ignited, continues fo till all 

 the oleaginous and fulphureous particles contained in the mountain are confumed. 



Volcanoes are of two kinds, contracted and dilated. The former are found where a 

 great quantity of inflammable matter is confined in fmall fpace ; the latter where thefe 

 combuftibles are fcattered at a confiderable difl:ance from one another. The 

 firft are chiefly contained in the bowels of mountains, which may be con- 

 fidered as the natural depofitaries of thefe fubfl:ances. The fecend may be 

 confidered as ramifications, which, though proceeding from the former, are, however, 

 independent, extending themfelves under the plains, and traverfing them in feveral 

 directions. This being admitted, it will appear, that in whatever country volcanoes, or 

 depofitaries of thefe fubftances, are very common, the 4)lains will be more diverfified 

 with thefe ramifications ; for v/e are not to imagine that it is only within the bofoms of 

 mountains that fubftances of this nature exift, and that they are not difleminated through 

 all the parts of the adjacent regfen. Thus the country now under confideration, abound- 

 ing in thefe igneous fubftances more than any other, muft by the continual inflammation 

 which neceflarily follows their natural preparation for it, be more expofed to earthquakes, 

 Befides the fuggeftions of natural reafon, that a country containing many volcanoes 

 muft alfo be every where veined with ramifications of correfpondent fubftances, it is con- 

 firmed by experiment in Peru ; where we find almoft univerfally mines of nitre, fulphur, 

 vitriol, fait, bitumen, and other inflammatory fubftances, which fufficiently confirm the 

 truth of thefe inferences. 



The foil both of Quito and Valles, particularly the latter, is hollow and fpongy, fo 

 as to be fuller of cavities and pores than is ufually feen in that of other countries, and 

 ^ confequently abounds with fubterranean waters. Befides which, as I fliall prefently 

 fhew more at large, the waters, from the ice continually melting on the mountains, 

 ^ being filtrated through thefe pores during their defcent, penetrate deep into the cavities 

 of the earth, and during their fubterranean courfe, moiften, and turn into a kind of pafte, 

 thofe fulphureous and nitrous fubilances ; and though they are not here in fuch prodi- 

 gious quantities as in volcanoes, yet they are fufficient, from their inflammatory quality, 

 ta rarefy the air contained in them, which, eafily incorporating itfelf with that confined 

 in the innumerable pores, cavities, or veins of the earth, comprefles it by its greater ex- 

 panfion, and at the fame time rarefies it by its heat ; but the cavities being too narrow 

 to admit of its proper dilatation, it ftruggles for a vent, and thefe efforts fhake all the 

 J" contiguous parts j till at laft, where it finds the leaft refiftance, it forces itfelf a paffagCj 

 which fometimes clofes again by the tremulous motion it occafions, and at others con- 

 tinues open, as may be feen in different parts of all thefe countries. When, on account 



erf 



