S56 Prof. Biscliof oji the Natural History of 



rassed in their station on GitaguapicJnncha^ by the snow sur- 

 rounding their signals. Now, for many years, no snow has 

 been found on the summit of this mountain. The inhabi- 

 tants of Popayan also have remarked, that the inferior limit of 

 the snow covering the Purace is gradually rising, whilst the 

 mean temperature has remained the same for the last thirty 

 years, whence Boussingault infers that the Purace is sinking 

 down. 



That masses thrown up in a state of igneous fusion sink 

 again by degrees, in consequence of their consolidation and con- 

 traction, cannot be doubted. But even if their elevation had 

 taken place in a solid state, yet the immense masses of the Andes 

 have risen from depths, where a pretty high temperature pre- 

 vails. Supposing the Andes to have risen 24,000 feet in 

 height, that part of them which is now at the level of the sea, 

 must have been before the elevation so many thousand feet be- 

 low it. This part brought, therefore, with itself from beneath, 

 a temperature which was '^-^^-^ = 470° F. higher than that 

 which existed at the level of the sea before the elevation. The 

 same holds good of each part of the Andes, in any depth, so 

 that every where in erupted masses the temperature surpassed 

 that of the adjacent rocks by 470° F. Whilst now these 

 masses gradually lost their surplus of heat, they were contracted. 

 But this cooling of these masses can, as far as they are within 

 the earth, only be affected by conduction, therefore a long pe- 

 riod will elapse for that purpose. That part of the Andes, which 

 is elevated above the surface of the earth, and is exposed to the 

 atmosphere, will of course cool a little more quickly. If the 

 bases of the rocks thrown up be at a great depth below the 

 surface, their contraction in consequence of their cooling may 

 be very considerable, and as the elevation of the A?ides is said 

 to be one of the latest, this cooling and contraction may conti- 

 nue even at the present time in that part which is within the 

 earth. It is therefore possible to conceive that these effects are 

 the cause of the frequent earthquakes in the Andes. 



Besides, there is nothing opposed to the hypothesis, that the 

 powers, whatever they may be, which produced so remarkable 

 a phenomenon as these elevations, may not even now operate 

 in a less degree, and occasion the earthquakes so frequent in 



