M. Boussingault's [Oct. 



upon it in a line with the axis of the pit, a cone would hi 

 formed above it, which would be more or less elevated, in 

 proportion to the depth of the pit. It is thus, I conceive, 

 that Cotopaxi, Tunguragua, Chimborazo, &;c., have been 

 formed. 



The elastic fluids, in opening a passage through the crust 

 of trachyte, after being broken, have formed a communica- 

 tion between the surface of the earth and considerable 

 empty spaces existing at a greater or less depth. We may 

 conceive, then, that the fragment raised at first have sunk, 

 and filled up these vacuities. Thus, in place of the cone 

 being elevated above the point of eruption, a concavity has 

 been produced at the surface of the earth. It is thus, I 

 suppose, that the crater of Rucupichincha, and the green 

 sulphur lake of Tuqueres have been formed. 



I consider, then, the appearance of the trachyte cones of 

 the Cordilleras, to have been posterior to the elevation of 

 the mass of the Andes. These are not, however, the most 

 recent elevations which have taken place in these moun- 

 tains. In the vicinity of the highest peaks, and, I may 

 cite in illustration, Cayambe, Antisana and Chimborazo, 

 we observe small hills, composed of fragments, but differing 

 distinctly from the trachyte. The rock is black porphyry. 

 Its basis, which encloses crystals of glassy felspar, is coloured 

 by pyroxene. The felspar crystals are often rare, and then 

 the rock appears like basalt. I have never, however, seen 

 peridote. Sometimes, however, this rock is compact and 

 prismatic; sometimes it is scoriform, and filled with cells. 

 It might be taken for lava, if it were diffused over sufficient 

 space; but then, it occurs always in pieces which scarcely 

 attain the size of the fist. This matter has evidently been 

 produced at a very recent period. 



At Chorrera de Pisque, near Ibarra, a beautiful collonade 

 maybe observed resting on alluviam. In the farm of Lysco 

 this rock, in its broken state, during its ejection has opened 

 a passage through the trachyte. This appearance Hum- 

 boldt considered on Antisana as a current. I have dis- 

 cussed, in another place, my reasons for differing, in opinion, 

 with my illustrious friend. 



The extinct volcano of Calphi, placed at the bottom of 

 Chimborazo, consists of this kind of basalt. We visited it 



