538 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 2, 



vaster dimensions likely to be exhibited by the craters formed in 

 other volcanic districts by eruptions of a still more violent and tre- 

 mendous character, of vi^hich well- authenticated reports have from 

 time to time been received. Take, for example, the eruption of 

 Coseguina in the GuK of Ponseca in Central America, in 1835, whose 

 ashes were thickly scattered to distances of 700 miles, while within 

 a radius of 25 miles the ground was covered by its dejections to the 

 depth of more than ten feet, houses and woods being buried in them ! 

 — or that of Sangay in the Cordillera of S. America (1842-43), whose 

 black ejected ashes covered the surrounding country to a distance of 

 twelve miles, in beds 300 and 400 feet thick (Sebastian Wiss) ; — or 

 that of Tunguragua, another volcano of Quito, whose eruptions in 

 1797, of mud, i. e. of ashes mixed with melted snow or the contents 

 of a crater-lake, filled valleys many miles in length and 1000 feet 

 wide to the depth of 600 feet ! — or that of L' Altar, another volcano 

 of Quito, which eruption, before the discovery of America, is said by 

 M. Boussingault to have lasted eight years, and to have covered an 

 extensive plain with the fragments of what was previously a vast 

 trachytic cone, higher than Chimborazo ; — or that of Tomboro in the 

 Island of Sumbawa, which in 1815, for four months continuously, 

 threw up scoriae and ashes in such abundance that they broke down 

 the roofs of houses /ori^/ ^''^^^ distant, and were carried more than 300 

 miles in sufficient quantity to completely darken the air at that dis- 

 tance, while the floating cinders to the westward of Sumatra formed 

 a mass 2 feet thick, many miles in extent, through which ships with 

 difficulty forced their way*. Let any one ask himself what must be 

 the size of the hollows (i. e. the craters) left by the forcible expul- 

 sion of these startling quantities of matter from the centre of a 

 volcanic mountain, and he will, I think, have no difficulty in per- 

 ceiving that the dimensions of the largest known craters — say of 

 three, five, or even more miles in diameter — craters such as the exter- 

 nal " rings " of Santorini, St. Jago, the Mauritius, and others, — are no 

 greater than what we should expect to result from such a process, 

 which is the evacuation and outward dispersion, in fact, of the whole 

 central mass of each mountain, and that the occasional occurrence 

 of eruptions of stupendous violence and productive of such vast 

 amounts of ejected matters being an undisputed fact, we have no 

 need to resort to the supposition of either the circular upheaval of 

 previously horizontal beds, or the subsidence or engulfment of 

 mountain-tops (the effondrement of French geologists), to account for 

 the production of these and similar craters. Of course where such 

 craters have been exposed to the action of the sea- waves and cur- 

 rents, as in volcanic islands, or of torrents of water proceeding from 

 the sudden melting of a covering of snow, perhaps of internal glaciers, 

 they may have been enlarged by degradation, as Sir C. Lyell sup- 

 poses t ; but I am here discussing their origiual formation. 



The great paroxysmal eruption of Vesuvius in the year 79 was 

 clearly of this tremendously explosive character. It is singular that 



^ Lyell, Principles, p. 464, &e. t Quart. Journ. Geol. Soc. vi. p. 207- 



