THE VOYAGE OF THE BEAGLE 373 



quite unconnected with the ordinary course of the weather. 

 In the cases of great volcanic eruptions, as that of Co- 

 seguina, where torrents of rain fell at a time of the year most 

 unusual for it, and " almost unprecedented in Central 

 America," it is not difficult to understand that the volumes 

 of vapour and clouds of ashes might have disturbed the 

 atmospheric equilibrium. Humboldt extends this view to 

 the case of earthquakes unaccompanied by eruptions; but I 

 can hardly conceive it possible, that the small quantity of 

 aeriform fluids which then escape from the fissured ground, 

 can produce such remarkable effects. There appears much 

 probability in the view first proposed by Mr. P. Scrope, that 

 when the barometer is low, and when rain might naturally 

 be expected to fall, the diminished pressure of the atmos- 

 phere over a wide extent of country, might well determine 

 the precise day on which the earth, already stretched to the 

 utmost by the subterranean forces, should yield, crack, and 

 consequently tremble. It is, however, doubtful how far this 

 idea will explain the circumstances of torrents of rain fall- 

 ing in the dry season during several days, after an earth- 

 quake unaccompanied by an eruption; such cases seem to 

 bespeak some more intimate connection between the atmos- 

 pheric and subterranean regions. 



Finding little of interest in this part of the ravine, we 

 retraced our steps to the house of Don Benito, where I stayed 

 two days collecting fossil shells and wood. Great prostrate 

 silicified trunks of trees, embedded in a conglomerate, were 

 extraordinarily numerous. I measured one, which was fif- 

 teen feet in circumference: how surprising it is that every 

 atom of the woody matter in this great cylinder should have 

 been removed and replaced by silex so perfectly, that each 

 vessel and pore is preserved ! These trees flourished at about 

 the period of our lower chalk; they all belonged to the fir- 

 tribe. It was amusing to hear the inhabitants discussing the 

 nature of the fossil shells which I collected, almost in the 

 same terms as were used a century ago in Europe, namely, 

 whether or not they had been thus " born by nature." My 

 geological examination of the country generally created a 

 good deal of surprise amongst the Chilenos : it was long 

 before they could be convinced that I was not hunting for 



