104 M. Boussingaults Account of 



should be the reverse of what it generally is under the tropics ; 

 that is, that the barometer, from nine o'clock in the morning to 

 four o'clock in the afternoon, had risen instead of falling. This 

 irregularity arose probably from an accidental circumstance ; I 

 am so much the more inclined to believe this, as I found that 

 there was the usual variation at the farm of Antisana, though it 

 was less considerable than in the plain. 



As we descended, an icy rain became mixed with the hail. 

 The night surprised us on our route; it was eight o'clock when 

 we arrived at the farm of Chimborazo. 



The geological observations which I was able to make during 

 this expedition, ail tend to confirm the ideas which I have in 

 another place expressed on the nature of the trachyte, which 

 forms the ridge of the Andes ; for on Chimborazo all the facts 

 were again presented to me which I have adduced in my de- 

 scription of the Equatorial volcanos. This is evidently an ex- 

 tinct volcano like Cotopaxi, Antisana, Tunguragua, and, in ge- 

 neral, all the mountains standing on the plateaus of the Andes. 

 Chimborazo consists of an assemblage of irregularly piled up 

 masses of trachyte. There, frequently, enormous blocks of 

 trachyte have been sent up from the volcano in a hard condi- 

 tion ; their edges are sharp ; nothing indicates that they have 

 once been in a melted or soft state. There is nowhere to be 

 seen on any of the volcanos an approach to a stream of lava. 

 Nothing is ejected from these craters but masses of mud, elastic 

 fluids, and red-hot fragments of trachyte, which have more or 

 less the character of scoriae, and are often thrown to very consi- 

 derable distances. 



A plateau forms the foot of Chimborazo, which may be stu- 

 died in detail in the rivulets near the farm. There I ascertained 

 that the trachyte is not at all arranged in strata, but is traversed 

 by fissures in all directions. This rock consists chiefly of fel- 

 spar, which is generally of a grey colour, and includes crystals 

 of augite and a glassy felspar. 



The trachyte rises towards Chimborazo, and often presents 

 considerable rents, which become deeper and broader as they 

 approach the mountain. One might say that Chimborazo, as it 

 rose, had burst the plateau which serves as its basis. 



The trachytic rock, which constitutes the greater part of the 



