HumboldCs Theory of the Volcano of Jorullo. 59 



which spread itself on all sides with great reluctance, and, 

 therefore, would necessarily remain thickest and deepest where 

 the lava was produced in greatest abundance, diminishing in 

 bulk from thence towards the limits of the space it covered ; 

 i. e. would assume precisely the convexity of form peculiar to 

 Malpais. The subsequent projections of loose and pulveru- 

 lent matter from the six craters, and principally from Jorul- 

 lo, will have increased that convexity, covering it with strata 

 of volcanic ashes and augite crystals, which were reduced to 

 the appearance of a black clay by mixture with rain water. 



The fact that the only visible lava current proceeds from 

 the crater of Jorullo, is a strong confirmation of this opinion ; 

 since it is at once obvious why this is seen, while those that 

 may have been emitted previous to the formation of the other 

 cones are concealed ; and it becomes also probable that this 

 promontory of lava is merely one extremity of the current of 

 Jorullo, which, dipping under the strata of ashes, probably 

 unites with the streams proceeding from the other apertures 

 to form the substratum of the whole convex plain. 



Thus there is no difficulty in accounting for the convexity 

 of the plain of Malpais by the effects of the most ordinary 

 volcanic phenomena ; let us see whether this supposition will 

 explain the other remarkable appearances it is said to exhibit. 



And here a fact, recorded by M. de Humboldt himself, not 

 only tends to confirm, but may be almost said to prove, the 

 correctness of the view I have taken of the nature of the 

 plain. He says that, in 1780, the temperature of the fissures 

 which penetrate the surface of the plain and its hillocks, was 

 so high, that a cigar might be lighted by plunging it to the 

 depth of a few inches into them. Now I think it impossible 

 to account for this without allowing the whole plain to have 

 consisted of lava in a state of incandescence immediately be- 

 neath its outer crust ; a circumstance to be expected, even so 

 much as 20 years after its emission, in a bed of lava more than 

 500 feet in thickness, since Hamilton observed of one of the 

 smaller currents of Vesuvius, that, three years after its pro- 

 duction, a stick might be inflamed by thrusting it into one of 

 the crevices of the rock. 



It remains only to account for the formation of the small 



