ON ITS EXTERIOR. VOLCANOES. 297 



rated, show yellow stains of oxide of iron. Singularly 

 enough, the soft bed of clay which unites the balls to- 

 gether has also a curved lamellar structure insinuating 

 itself into all the interstices between the balls. At 

 first sight I asked myself whether the whole, instead 

 of consisting of weathered basaltic balls containing a 

 little olivine, might not be masses in which a formative 

 process had been disturbed while taking place. This 

 view is opposed by the analogy of the actual hills of 

 basaltic balls interspersed with beds of clay and 

 marl which are found, often of very small dimensions, 

 in the Mittelgebirge of Bohemia, sometimes isolated, 

 and sometimes crowning both extremities of long 

 basaltic ridges. Some of the hornitos are either so 

 loosely cemented, or have such considerable internal 

 cavities, that, when mules are urged to pass over the 

 flatter ones amongst them, the animals' fore feet sink in, 

 whereas, in similar trials, the hills constructed by 

 termites (ants) do not give way. 



I did not find any scoriae, or fragments of older rocks 

 which had been broken through, enveloped in the basalt 

 of the hornitos, as was the case in the lavas of the great 

 Jorullo. The circumstance which particularly justifies 

 the name of hornos, or hornitos (ovens), is that the smoke 

 issues from them, not at the top, but at the sides. (I speak 

 of the time when I saw them and wrote in my journal, 

 Sept, 18, 1803.) In 1780 it was still possible to light 

 cigars at them by fastening the cigar to a stick pushed 

 in two or three inches deep ; in some places the air was 

 so much heated by their neighbourhood, as to oblige the 

 passer-by to turn out of his way. Notwithstanding the 

 cooling-down which, according to the unanimous testi- 



