TRUE VOLCANOES. 299 



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 one to three feet. The black basaltic mass is 

 penetrated by hot vapors and broken up into an earthy form, 

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

 when detached, exhibit yellow spots of oxyd 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 olivin, 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 globu- 

 lar 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, while in similar experiments 

 which I made the hills constructed by the termites resisted. 

 In the basaltic mass of the hornitos I found no immersed 

 scorice, 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 Tlornitos is especially justified by the cir- 

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

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

 18th of 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 two or three 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 which, 

 according to the universal testimony of the Indians, the district 

 had undergone within 20 years, I found the temperature in 

 the fissures of the hornitos to range between 199 and 203 ; 

 and at a distance of twenty feet from some hills the tempera- 

 ture of the air was still 108-5 and 116'2, at a point where 

 no vapors reached me, the true temperature of the atmos- 

 phere of the Playas being at the same time scarcely 77. 

 The weak sulphuric vapors decolorized strips of test paper, 

 and rose visibly, for some hours after sunrise, to a height of 



