CHAP. II. 



Terceira. 29 



soon blackens iron, and is of much too high temperature 

 to be endured by the hand. The manner in which the 

 solid trachyte is changed on the borders of these orifices 

 is curious: first, the base becomes earthy, with red 

 freckles evidently due to the oxidation of particles of 

 iron ; then it becomes soft ; and lastly, even the crystals 

 of glassy feldspar yield to the dissolving agent. After 

 the mass is converted into clay, the oxide of iron seems 

 to be entirely removed from some parts, which are left 

 perfectly white, whilst in other neighbouring parts, 

 which are of the brightest red colour, it seems to be 

 deposited in greater quantity ; some other masses are 

 marbled with the two distinct colours. Portions of the 

 white clay, now that they are dry, cannot be distinguished 

 by the eye from the finest prepared chalk ; and when 

 placed between the teeth they feel equally soft-grained ; 

 the inhabitants use this substance for white-washing 

 their houses. The cause of the iron being dissolved in 

 one part, and close by being again deposited, is obscure; 

 but the fact has been observed in several other places. 1 

 In some half-decayed specimens, I found small, globular, 

 aggregations of yellow hyalite, resembling gum-arabic, 

 which no doubt had been deposited by the steam. 



As there is no escape for the rain-water, which 

 trickles down the sides of the ravine-like hollow, whence 

 the steam issues, it must all percolate downwards 

 through the fissures at its bottom. Some of the in- 

 habitants informed me that it was on record that 

 flames (some luminous appearance?) had originally 

 proceeded from these cracks, and that the flames had 



1 Spallanzani, Dolomieu and Hoffman have described similar 

 cases in the Italian volcanic islands. Dolomieu says the iron at the 

 Panza Islands is redeposited in the form of veins (p. 86, ' Meruoire 

 sur les Isles Ponces '). These authors likewise believe that the steam 

 deposits silica : it is now experimentally known that vapour of a 

 high temperature is able to dissolve silica. 



