an Attempt to ascend Chimborazo. 105 



province of Quito, presents few variations. The confused heaped 

 blocks, which form the volcanic cones, are of the same mineralo- 

 gical nature as the rock which is their foundation. These cones 

 and steep mountains were doubtless elevated by elastic fluids, 

 at the points where there was the least resistance. The trachytic 

 rock, which is shattered into innumerable fragments, has, as it 

 was brought to the surface, been elevated by the vapours which 

 were evolved. After the eruption, the shattered rock must have 

 acquired a larger volume, as all the masses could not return to 

 the place whence they came ; they, therefore, were heaped over 

 the opening through which the evolution of the gas had taken 

 place. 



It is precisely what would happen if we were to dig a well in 

 a hard and compact rock, and then wished to throw in the 

 masses of stone thus obtained ; the well would soon be filled, 

 ancQif we were to continue heaping up the stones in the same 

 direction, a cone would be formed over the mouth of the well, 

 which would be higher the deeper the excavation. In this man- 

 ner, I think, Cotopaxi, Tunguragua, Chimborazo, &c. have 

 been formed. 



The elastic fluids, which, after they had broken up the tra- 

 chytic crust, opened through it a passage for themselves, might 

 on the surface of the ground unite into more or less considerable 

 cavities, hollows already in existence ; and we can thus conceive 

 that the masses of rock, which were at first raised, might after- 

 wards sink and occupy these hollows. Thus a cavity must have 

 been formed on the surface, in place of a cone elevated on the 

 point of eruption. I can thus understand the wonderful sink- 

 ings presented by the crater of Rucuipichincha, and also the 

 green lake of the solfatara of Tuqueres, of which I have given, 

 in another memoir, a full description. 



I consider, then, the formation of the trachytic cones of the 

 Cordilleras as posterior to the elevation of the mass of the Andes. 

 But these cones are not the newest elevations which occur in 

 these mountains. In the neighbourhood of the highest peaks, 

 namely, those of Cayambe, Antisana, and Chimborazo, small 

 mountains are to be observed, consisting of masses of rock of a 

 newer description, and differing distinctly from the usual tra- 

 chyte. It is black, porphyritic, and its basis, which includes 



