TRUE VOLCANOES. 317 



clusively on the western side of the great volcano, as indeed, 

 the eastern part towards the Cerro de Cuiche, scarcely con- 

 stitutes aV^ f the entire area of the vesicular elevation of 

 the Playas. Each of the numerous hornitos is composed of 

 weathered basaltic spheres, with fragments separated like 

 concentric shells ; I was frequently able to count from 24 to 

 28 such shells. The balls are flattened into a somewhat 

 spheroidal form, and are usually 15 18 inches in diameter, 

 but vary from 1 to 3 feet. The black basaltic mass is pene- 

 trated by hot vapours and broken up into an earthy form, 

 although the nucleus is of greater density, whilst the shells, 

 when detached, exhibit yellow spots of oxide of iron. Even 

 the soft, loamy mass which unites the balls is, singularly 

 enough, divided into curved lamellae, which wind through 

 all the interstices of the balls. At the first glance I asked 

 myself whether the whole, instead of weathered basaltic 

 spheroids, containing but little olivine, did not perhaps pre- 

 sent masses disturbed in the course of their formation. But 

 in opposition to this we have the analogy of the hills of glo- 

 bular basalt, mixed with layers of clay and marl, which are 

 found, often of very small dimensions, in the central chain of 

 Bohemia, sometimes isolated and sometimes crowning long 

 basaltic ridges at both extremities. Some of the hornitos 

 are so much broken up, or have such large internal cavities, 

 that mules when compelled to place their fore-feet upon the 

 flatter ones, sink in deeply, whilst in similar experiments 

 which I made, the hills constructed by the termites, re- 

 sisted. 



In the basaltic mass of the hornitos I found no immersed 

 scoria?, or fragments of old rocks which had been penetrated, 

 AS is the case in the lavas of the great Jorullo. The appel- 

 lation Hornos or Hornitos is especially justified by the cir- 

 cumstance that in each of them (I speak of the period when 

 I travelled over the Playas de Jorullo and wrote my journal, 

 18 September, 1803,) the columns of smoke break out, not 

 from the summit, but laterally. In the year 1780, cigars 

 might still be lighted when they were fastened to a stick 

 and pushed in to a depth of 2 or 3 inches ; in some places 

 the air was at that time so much heated by the vicinity of 

 the hornitos, that it was necessary to turn away from 

 one's proposed course. Notwithstanding the refrigeration 



