2vV REACTION OF THE INTERIOR OF THE EARTH 



high, over which the waters of the brook, which had be- 

 come heated (the Eio de San Pedro), fall. The best ex- 

 amination which I was thus enabled to make of the face of 

 the precipice showed black horizontal beds of mud or clay, 

 intermixed with sand or rapilli. At other points, which 

 I had not seen, Burkart observed " at the perpendicular 

 edge of the upheaved ground, where it is difficult of 

 ascent, a light-grey, much-weathered, not dense basalt 

 with many grains of olivine." ( 433 ) This accurate and 

 experienced observer ( 434 ), when on the spot, imbibed, 

 as I had done, the impression of a blistering of the 

 surface by elastic vapours, contrary to the opinion of 

 celebrated geologists ( 435 ), who ascribed the convexity, 

 found by me by direct measurement, solely to the greater 

 mass of the outpoured lava more immediately around 

 the foot of the volcano. 



The many thousand small cones of eruption (or more 

 strictly of a roundish or oblong shape, like a baker's 

 oven), which are pretty equably distributed over the 

 upheaved surface, are generally about from four to ten 

 feet high. Much the greater number are on the west 

 side of the great volcano ; the eastern portion, towards 

 the Cerro de Cuiche, being scarcely one twenty-fifth of the 

 whole upheaved area of the playas. The hornitos are 

 composed of weathered balls of basalt, consisting of 

 concentric shells, of which I often counted as many as 

 twenty-four or twenty-eight. The balls are rather sphe- 

 roids than spheres, generally from sixteen to nineteen 

 inches in diameter ; but there are also many others vary- 

 ing between one foot and three feet. The black basalt is 

 pervaded by hot vapours, and has become earthy ; but 

 the nucleus is more dense, while the shells, when sepa- 



