348 THE POPULAR SCIENCE MONTHLY. 



the crater beneath. Some of tbe volatile products of volcanoes are ci 

 economical value, like the sal-ammoniac, sulphur, and boracic acid of 

 Vulcano, which have given rise to extensive chemical works. 



Solid substances are ejected and accumulate around the orifices, 

 where they frequently form large mountains. They are fragments of 

 rock torn from the formations through which the eruptive stream 

 passes crystallized minerals, or matters which, derived from sources 

 far below the earth's surface, issue in an incandescent or molten con- 

 dition, and to which the name of lavas is properly applied. The lavas 

 are composed of the silicates of aluminium, magnesium, calcium, iron, 

 sodium, and potassium in different degrees of combination or mixture. 

 Oxygen, in silicic acid or in a metallic oxide, makes up nearly half the 

 weight of all of them, the metalloid silicon one fourth, and aluminium 

 one tenth of the most of them. Silica or silicic acid rock-crystal or 

 flint is present in proportions varying from one half to four fifths of 

 the whole mass. Those lavas in which silica is present in larger pro- 

 portion are called " acid lavas " ; those in which the base is in greater 

 proportion, " basic lavas." Between these are the "intermediate lavas," 

 in which the proportion of silica is lower than in the acid lavas, and 

 the proportion of bases is lower than in the basic lavas. Of the five 

 great groups into which geologists have divided the lavas, the rhy- 

 olites are acid, the basalts are basic, and the trachytes, andesites, and 

 phonolites are intermediate. 



The structure of lavas can be more clearly ascertained by studying 

 them in the condition of transparent or semi-transjiai-ent slices under 

 the microscope. Most of them are made up of crystals of different 

 minerals, varying in size from those which are hardly visible to the 

 naked eye to those of an inch or more in length. Others appear glassy 

 in structui'e. Under the microscope, they are shown to be made up of 

 two kinds of materials, a base or ground-mass of a glassy character, 

 and distinct crystals irregularly distributed through this glassy base, 

 like the raisins in a cake. In some cases the vitreous part makes up 

 the whole mass of the rock ; in others smaller or larger numbers of 

 crystals are seen to be scattered through a glassy base ; while in others 

 again the crystals are so numerous that the presence of an intervening 

 vitreous ground-mass can be detected ox\\j by the aid of the micro- 

 scope. When slices of the glassy materials are examined with high 

 powers, cloudy patches are seen diffused through the substance, which 

 under still higher powers resolve themselves into innumerable particles 

 having very definite outlines, some transparent and some opaque. At 

 the same time fresh cloudy patches are brought into view, requiring 

 still higher powers for their resolution ; and so the process may go on, 

 as it does in examining the nebulae with the telescope. The minute 

 particles thus brought into view are called microliths or crystallites. 

 Sometimes, instead of being indiscriminately diffused, they are col- 

 lected in groups of very definite form, resembling the frost-work on 



