^o SECRETS OF EARTH AND SEA 



glacier in appearance. A foot below the surface it was 

 still red-hot, and a stick pushed into a crevice caught fire. 

 It was not safe to venture far on to the pie-crust surface. 

 A couple of miles away the campanile of the church of a 

 village called Massa di Somma stood out, leaning like 

 that of Pisa, from the petrified mass, whilst the rest of the 

 village was overwhelmed and covered in by the great 

 stream. 



The curious resemblance of the lava-stream to a glacier 

 arose from the fact that it was almost completely covered 

 by a white snow-like powder. This snow-like powder, which 

 often appears on freshly-run lava, is salt — common sea 

 salt and other mineral salts dissolved in the water ejected 

 as steam mixed with the lava. The steam condenses, as 

 the lava cools, into water and evaporates slowly, leaving 

 the salt as crystals. Often these are not white, but con- 

 tain iron salt, mixed with the white sodium, potassium, 

 and ammonium chlorides, which gives them a yellow or 

 orange colour. Salts coloured in this way have the 

 appearance of sulphur, and are often mistaken for it. 

 The whole of the interior of the crater of Vesuvius 

 when I revisited it in 1875 was thus coloured yellow, 

 and I have a water-colour sketch of the scene made 

 by a friend who came with me for the purpose. As 

 a matter of fact, though small quantities of the choking 

 gas called "sulphurous acid" are among the vapours 

 given off by Vesuvius, there is no deposit of sulphur 

 there. Some large volcanoes (in Mexico and Japan) 

 have made deposits of sulphur, which are dug for com- 

 mercial purposes ; but the sulphur of Sicily is not, and has 

 not been, thrown out or volatilized by Etna. It occurs in 

 rough masses and in splendid crystals in a tertiary cal- 

 careous marine deposit, and its deposition was probably 

 due to a chemical decomposition of constituents of the sea 

 water brought about by minute plants, known as " sulphur 



