434 



PHYSICAL GEOGRAPHY. 



small scale, by burying in the ground a moistened mixture of sulphur and iron filings, 

 when the mass becomes gradually heated, takes fire, and explodes. The lava which 

 flows out to the surface in volcanic eruptions, or is driven up in dust and scoriae, is owing 

 to the violent extrication, through a vent, of the steam which has been generated, 

 accumulated, and confined, the oscillations and heavings of the ground in earthquakes 

 being produced by the action of elastic vapours and gases endeavouring to effect their 



escape by a rending of the strata. Such 

 is the hypothesis. It requires the metal 

 loids to exist in the interior of the earth, 

 and the admission of water in sufficient 

 abundance to them, a condition which may 

 be conceded. But we have choice of 

 another theory, that of Central Heat, 

 which, upon the whole, is more probable, 

 and has received greater countenance. It 

 supposes the interior of the globe, at a 

 varying distance from the surface, to be in 

 a state of actual incandescence j and it is 

 a well-attested fact, that everywhere a 

 higher temperature is met with, in propor- 



Volcano of Orizaba. tlon to the depth to which the CrUSt of 



the earth is perforated. Water, gaining access to the heated interior mass, through fissures, 

 generates steam and other gases, which, struggling to disengage themselves, produce 

 tremors of the surface, or earthquakes, while occasionally they find their way to open 

 vents, and signalise their liberation by the ejection of volcanic products. 



It is difficult to form just views of events occasioning such calamities to the human race 

 as the reduction to instability of the before fixed and firm foundations of the globe. When 

 in a few passing seconds peaceful homes become the sepulchres of their inhabitants, and 

 the roof that long has sheltered them from inclement elements is the engine of their 

 destruction when scenes verdant through the industry of man are converted into 

 frightful desolations, and cities fall, involving youth, beauty, and innocence in indiscriminate 

 ruin with proficient and inveterate vice men are prone to reflections questioning the 

 goodness and fitness of things, challenging the arrangements of the Creator in the scheme 

 of the creation. Yet in most cases this is the offspring of a miserable selfishness ; for 

 the same parties will gloat over a battle in which their nation has been victorious, though 

 destructive to more than ever perished by any one natural visitation of earthquake, 

 volcano, or pestilence ; the human action, at the same time, involving a deep moral guilt 

 which belongs not to the physical phenomena. It may be well to recollect, in relation to 

 these paroxysms of nature, that science and philosophy step in, and suggest relieving 

 considerations. They unfold the long antiquity of the earth, teach us to contemplate it 

 in connection with an era compared to which an age is a span, and unfold the tendency 

 of those milder agencies which are in incessant action upon it, and which, though slow 

 workers, would effect extensive and disastrous changes in the succession of centuries, if 

 there was no counteraction to them. Inequalities of the surface eminently adapt the 

 globe to be the residence of man during his threescore years and ten, and of the myriads 

 of different races of beings that inhabit it. But the waste of the elevated dry land is a 

 gradual yet sure effect produced by the atmospheric and aqueous causes that constantly 

 act upon it. These, without an antagonist power, would, in time, reduce the inequalities 

 of alluvial countries nearly to a uniform level, bring the habitable part of our planet down 

 to the ocean line, and convert scenes of fertility and busy life into vast lagunes and 



