1859.] SCROPE COKES AlH) CEATEES. 535 



more recently active volcanic districts of Italy. MM. Yirlet and 

 Hoffman also joined in this recantation, so honourable to these 

 geologists. 



Graters. — I have hitherto confined my remarks almost entirely to 

 the upheaval-theory as it bears upon the mode of formation of vol- 

 canic cones or mountains. There remains something to be said 

 upon the idea which it involves as to the generation of volcanic 

 craters. 



Craters are perhaps the most characteristic features of volcanic 

 districts; they are met with of all dimensions, from the slight 

 saucer-like depressions at the summit or on the flank of a cone of 

 scoriae, to the wide and often deep caldron circled by more or less 

 sloping or precipitous banks, and the broken range of concentrically 

 disposed chffs, the remaining segments of what was once probably 

 a complete circular or elliptical basin of vast horizontal extent. All, 

 however, are alike designated as volcanic craters by geologists of 

 every school, the only dispute being confined to their mode of origin. 

 The upheavalists look upon them aU, I believe without exception, 

 and whatever their size or shape, as the contemporaneous result of 

 that process to which they attribute the elevation of the heights 

 which surround them, viz. the single, sudden, and violent outbreak 

 of a great volume of vapour from beneath the surface. They com- 

 pare the phenomenon to the explosion of a mine, or the bursting 

 of an enormous subterranean bubble or blister, which, swelling 

 upward, first upheaves the overlying strata, then opens at its apex, 

 leaviug them tilted around the fractured edges of the cavity. These 

 are, indeed, the precise words of M. de Humboldt, which I quoted in 

 the early portion of this paper. This theory seems necessarily to 

 involve, particularly in the case of the larger craters, the notion of 

 the subsidence or foundering of the central parts of the upheaved 

 mass into the gulf so opened. M. Dufrenoy expressly attributes to 

 subsidence of this kind after explosion, the small craters of the para- 

 sitic cones of Vesuvius, which are in his view bell-shaped bubbles that 

 have burst at the apex. So, too, M. de Beaumont refers the forma- 

 tion of the great crater of the Yal del Bove in Etna to the foundering 

 of the roof or crust of the colossal bubble whose sudden upheaval 

 created Etna, and whose bursting, he says, must have been to any 

 modern eruption what the blowing-up of a powder-magazine is in 

 comparison mth the firing of a pistol*. 



I will not repeat here the argument already referred to, that any 

 such single explosive shock could not possibly tilt up solid strata in 

 an unbroken ring, which is the form of the greater number of 

 craters. I content myself with declaring my conviction that such a 

 phenomenon as is here assumed as the origin of every crater is a 

 pure imagination, nothing like it having ever been witnessed by cre- 

 dible observers, and is, indeed, wholly different from what is observed 

 whenever and wherever a great volcanic eruption takes place and a 

 crater is produced by it. 



* P. 192, Memoire, &c., Mt. Etna. 



