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temperature. This has a tendency to loosen the im- 

 perfectly connected masses of rock thus exposed and 

 to facilitate the admission of water, during heavy 

 rains, into the crevices. When this occurs at the tem- 

 perature of the freezing point, the water is congealed, 

 and by its expansion, rocks of an enormous size, as well 

 as smaller masses, have often been broken up from 

 their lofty situations and tumbled to the mountain's 

 base. This being annually repeated, through a lapse 

 of ages, occasions, in numerous instances, an immense 

 accumulation of debris at the foot of the mountains, 

 which, by the annual growth and decomposition of 

 vegetable substances, and the sand and dust, brought 

 by the winds and deposited in the interstices, assumes 

 somewhat of an uniform appearance, such, at least, as 

 to induce a belief that the whole mass thus accumula- 

 ted is the result of decomposition. 



But I can in no wise, view it as such. Were we to 

 explore those new formed districts, by cutting a hori- 

 zontal drift about midway up their sides, we should find 

 the fallen masses lying, like the ruins of the ancient 

 cities of Egypt, in promiscuous disorder, as unchanged 

 in form and texture almost as when detached from the 

 parent rock. This, therefore, cannot be called the 

 result of actual decomposition; but rather the dis- 

 ruption, or breaking up of rocks by accidental causes, 

 which are by no means common to all. 



It now remains to consider the nature, causes, and 

 extent, of the decomposition of rocks. 



