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THEORIES OF HUMBOLDT AND LYELL. 



thousands of flattish conical mounds, from six to nine feet high, 

 which as well as large fissures traversing the plain, acted as fume- 

 roles, giving out clouds of sulphuric acid and hot aqueous vapor. 

 The two small rivers before mentioned disappeared during the erup 

 tion, losing themselves below the eastern extremity of the plain, and 

 reappearing as hot springs at its western limit. Humboldt attributed 

 the convexity of the plain to inflation below ; supposing the ground, 

 for four square miles in extent, to have risen in the shape of a blad- 

 der to the elevation of 550 feet above the plain in the highest part. 

 But this theory is by no means borne out by the facts described ; 

 and it is the more necessary to scrutinize closely the proofs relied 

 on, because the opinion of Humboldt appears to have been received 

 as if founded upon direct observation, and has been made the 

 ground work of other bold and extraordinary theories. Mr. Scrope 

 has suggested that the phenomena may be accounted for far more 

 naturally by supposing that lava flowed simultaneously from the dif- 

 ferent orifices, and principally from Jorullo, united with a sort of 

 pool or lake. As it poured forth on a surface previously flat, it 

 would, if its liquidity was not very great, remain thickest and 

 deepest near its source, and diminish in bulk from thence towards 

 the limits of the space which it covered. Fresh supplies were 

 probably emitted successively during the course of an eruption 

 which lasted a year; and some of these, resting on those first 

 emitted, might only spread to a small distance from the foot of the 

 cone, where they would necessarily accumulate to a great height. 



" The showers, also, of loose and pulverulent matter from the six 

 craters, and principally from Jorullo, would be composed of heavier 

 and more bulky particles near the cones, and would raise the ground 

 at their base, where, mixing with rain, they might have given rise 

 to the stratum of black clay which is described as covering the lava. 



" The small conical mounds called £ hornitos J or little ovens may 

 resemble those five or six small hillocks which existed in 1823 on 

 the Vesuvian lava, and sent forth columns of vapor, having been 

 produced by the disengagement of elastic fluids heaving up small 

 dome-shaped masses of lava. The fissures mentioned by Humboldt 

 as of frequent occurrence, are such as might naturally accompany 

 the consolidation of a thick bed of lava, contracting as it congeals ; 

 and the appearance of rivers is the usual result of the occupation 

 of the lower part of the valley or plain by lava, of which there are 

 many beautiful examples in the old lava currents of Auvergne. The 

 heat of the c hornitos ' is stated to have diminished from the first ; 

 and Mr. Bullock, who visited the spot many years after Humboldt, 



