118 Volcanos. 



the general expansive force remains invariable. If not, this force 

 increases, and must at length prevail over the united forces of re- 

 pression, and produce elevations of the superficial strata, earth- 

 quakes, &c. The author thus distinguishes between the general 

 or primary, and the local or secondary expansive forces, each 

 having their peculiar force : the first residing in the general sub- 

 terranean bed of heated rock ; the second in minor and less deep- 

 ly seated foci. The laws thus determined hold good, whatever 

 the scale of magnitude of the phenomena they give rise to, 

 whether the elevation of a few square yards of rock, or of a whole 

 continent; a quiescent interval of a few hours, or of centuries. 



" Every habitual volcano acts, therefore, as a safety-valve to 

 the globe, the caloric which emanates from its interior passing 

 off by means of this vent into outer space. But these eruptions 

 are necessarily accompanied by circumstances tending to impede 

 their continuance, and they are thus, in the generality of cases, 

 rendered intermittent. Where the opposing forces of expansion 

 and repression are in equilibrio, the volcano is in the first of the 

 phases noticed above. Where they oscillate frequently about an 

 equilibrium, in the second. Where the oscillations are on a large 

 scale, in the third. The first must necessarily be very rare. In the 

 instance of Stromboli, our author attributes the permanence of its 

 eruption solely to the peculiar form of the crater; the aperture 

 of the volcano having a high and sloping ridge only on one side ; on 

 the other a precipitous slope down to the sea, which is there un- 

 fathomable. Owing to this remarkable figure, less than one half 

 of the scoriae projected from the aperture at each explosion fall 

 again into it, and, consequently, there can be no accumulation of 

 fragments on the surface of the lava within the vent, which al- 

 ways remains level, or nearly so, with the mouth of this aper- 

 ture, without being discharged otherwise than in fragments tossed 

 up by the bubbles of vapor which escape from it. The volcano 

 of Bourbon again, a similar example of almost continual eruption, 

 is shown by the author to owe this character to another peculia- 

 rity of form. This volcanic mountain is a complete obtuse cone, 

 and there exists at the apex an almost permanent source of a 

 very fluid and glassy lava, which slowly boils over the lips of the 

 circular orifice, and flows rapidly on all sides down the steep slopes 

 of the cone. Thus, in this, as in the former instance, the force 

 of repression remains fixed, and that of expansion being always 

 slightly in excess, the eruption is permanent. Where, however, 

 these forces are so nearly in equilibrio, a very slight addition to 

 that of repression may stop the eruption for a certain time, and 

 Mr. P. S. supposes that even changes in the density of the at- 

 mosphere will occasionally produce this effect ; and that a per- 

 manently active volcano, whose phenomena are, according to him, 



