LIMITS AND SPREAD OF FEVERS. 381 
demies of 1800 and 1804. I examined with attention the 
bed of the torrent of La Guayra ; and found it to consist 
merely of a barren soil, blocks of mica-slate, and gneiss, 
a. ■ tabling pyrites detached from the Sierra de Avila, but 
nothing that could have had any effect in deteriorating the 
purity of the air. 
Since the years 1797 and 1798, at which periods there 
prevailed dreadful mortality at Philadelphia, St. Lucia, and 
St. Domingo, the yellow fever has continued its ravages at 
La Gruayra. It has proved fatal not only to the troops 
newly arrived from Spain, but also to those levied in parts 
remote from the coasts, in the llanos between Calabozo 
and Uritucu, regions almost as hot as La Guayra, but 
favourable to health. This latter fact would seem more 
surprising, did we not know, that even the natives of Vera 
Cruz, who are not attacked with typhus in their own town, 
sometimes sink under it during the epidemics of the Ha- 
vannah and the United States. As the black vomit finds 
an insurmountable barrier at the Encero (four hundred 
and seventy-six toises high), on the declivity of the moun- 
tains of Mexico, in the direction of Xalapa, where oaks 
begin to appear, and the climate begins to be cool and 
pleasant, so the yellow fever scarcely ever passes beyond 
the ridge of mountains which separates La Guayra from the 
valley of Caracas. This valley has been exempt from the 
malady for a considerable time ; for we must not confound 
the vomito and the yellow fever with the irregular and 
bilious fevers. The Cumbre and the Cerro de Avila form a 
very useful rampart to the town of Caracas, the elevation of 
which a little exceeds that of the Encero, but of which the 
uean temperature is above that of Xalapa. 
I have published in another work* the observations made 
by M. Bonpland and myself on the locality of the towns 
periodically subject to the visitation of yellow fever ; and I 
shall not hazard here any new conjectures on the changes 
observed in the pathogenic constitution of particular locali- 
ties. The more I reflect on this subject, the more mysterious 
appears to mo all that relates to those gaseous emanations 
which we call so vaguely the seeds of contagion, and which 
are supposed to be developed by a corrupted air, destroyed 
* Nouvelle Espagr.e, tom. ii. 
