I VOLCANIC PRODUCTS. 277 



! tured by Sir H. Davy, that the earths and alkalies which form lavas, 

 exist in the centre of the globe in a metallic state, and take fire by 

 the access of water. The property of the newly discovered metals 

 to inflame instantly on the access of water offers an easy explanation 

 i of the origin of volcanic fires, could we suppose that substances so 

 \ extremely inflammable and oxidable have remained for ages in a 

 j metallic state. This theory is now abandoned. There may, how- 

 i ever, be processes going on in the vast laboratory of the globe, that 

 separate the earths from oxygen, and prepare them for the support 

 I of volcanic fires, by which they are thrown upon the surface, and 

 j thus establish a communication between the internal and external 

 I parts of our planet. 



j The saline products of volcanoes are not numerous. The sul- 



i phureous and sulphuric acids, formed by the combustion of sulphur 

 during eruptions, act upon lavas and rocks, and produce different 

 combinations, of which the most important are alum, sulphate of 

 magnesia, sulphate of iron, or green copperas, and gypsum. Muri- 



1 ate of ammonia, or sal-ammoniac, forms an incrustation on many la- 

 vas soon after they cool : muriate of soda, or common salt, and mu- 

 riate of copper and of iron, are found in the craters of volcanoes. 

 Muriatic acid, in an uncombined state, occurs in some of the spongy 

 lavas in Auvergne. 



The principal metallic substances in volcanic rocks are iron and 

 titanium ; but ores of antimony, copper and manganese, have some- 



: times been found in the craters of volcanoes. Tellurium, gold, and 

 mercury are also said to occur in some volcanic rocks. The island 

 of Ischia, which is entirely volcanic, contains a mine of gold. 



Iron, in the form of brilliant laminae, called specular iron, occurs 

 in the cavities and fissures of many lavas. Magnetic iron ore, and 

 oxide of iron, with iron sand and titanium, form a constituent part of 

 nearly all dark-coloured lavas or basalt. 



The earthy products of volcanoes are either vitreous, or stony, or 

 scoriaceous, or spongy, or in loose grains or powder. Volcanic 

 rocks are composed chiefly of felspar, and the dark-coloured min- 

 eral called augite ; they contain, also, hornblende and grains of mag- 

 netic iron ore, with titanium and iron sand, and the mineral called 

 olivine. Mica, leucite, iron pyrites, garnets, rubies, and zircon, are 

 also found in some volcanic rocks. The different states of lava, 

 whether vitreous, compact, or scoriaceous, depend on the different 

 circumstances under which it has cooled. 



Volcanic rocks, being composed principally of the two minerals, 

 felspar and augite, very minutely intermixed, derive their principal 

 characters from the prevalence of one or other of these minerals. 

 Those lavas in which felspar greatly predominates, have, generally, 

 a whitish or greyish colour, and melt into a white glass. The lavas 

 which contain a large portion of augite, have a dark colour, and melt 

 into a black glass. According to M. Cordier, all volcanic rocks thai 



