1835.] Ascent of Chimhorazo. 269 



repeated on Chimborazo which I had previously pointed 

 out in treating of the equatorial volcanoes. It is obviously 

 an extinct volcano. Like Cotopaxi, Antisana, Tunguragua, 

 and the mountains which stand, on the plateaus of the 

 Andes, the mass of Chimborazo is formed by the accumu- 

 lation of the debris of trachyte huddled together without 

 any order. These fragments, often of immense size, have 

 been upheaved in the solid state ; their angles are always 

 broken off, and there is no indication of their having been 

 in a state of fusion, or even softened. There is no appear- 

 ance in any of the equatorial volcanoes of a current of lava. 

 Nothing has been discharged from these craters, but impure 

 substances, elastic fluids, or incandescent blocks of tra- 

 chyte, more or less scorified, which have been often pro- 

 pelled to great distances. 



The base of Chimborazo is formed of a plateau, which 

 may be carefully studied in the torrent near the farm. Here 

 I could distinctly observe that the trachyte was not strati- 

 fied. This rock has a felspar basis, generally of a gray 

 colour, containing pyroxene and crystals of semi-vitreous 

 felspar. The trachyte rises up towards Chimborazo. It 

 often presents considerable crevices, which are larger and 

 deeper, in proportion to their propinquity to the mountain. 

 Chimborazo may be said to have caused the plateau on 

 which it is placed to rise up when it was formed. 



The trachyte which forms the greater portion of the pro- 

 vince of Quito presents little variety. The heaped up blocks 

 which form the volcanic cones, are similar, in their minera- 

 logical characters, to the rocks which constitute their bases. 

 These cones, and steep mountains, have, undoubtedly, 

 been raised by elastic fluids, which burst forth where there 

 was least resistance. The trachyte raised by the elastic 

 fluids, and broken into an infinite number of fragments, 

 has fallen on the surface. After the eruption, the fractured 

 rock necessarily occupied a greater space ; all the fragments 

 could not be returned to the orifice from whence they were 

 ejected, and have consequently been heaped up under it. 

 This is precisely what would happen, if, after having formed 

 a deep pit in a hard and compact rock, we wished to fill it 

 with the materials extracted from it ; whenever the excava- 

 tion was filled, while we continued to heap up the matter 



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