I must caution you, however, not to think that the above theory of volcanoes is accepted by all scientific men. On the contrary, there are many who believe that the earth is solid throughout because it has completely lost its original heat; that it is only comparatively small areas that are to be found filled with molten or at least highly heated material. But these opinions are held largely by those who have given their attention almost entirely to the phenomena of earthquakes, or who base their reasonings on mathematical grounds only and have not sufficiently considered the phenomena of volcanoes. Since, however, they can be better understood after we have explained the phenomena of earthquakes, we will defer their discussion to the last chapters of this book. CHAPTER XX SOME FORMS OF LAVA In describing the wonders of volcanoes, we must not fail to say something of the many remarkable forms that lava is capable of assuming. All volcanic lavas contain large quantities of an acid substance known as _silica_, or what is known better as _quartz sand_. This material exists in lava combined chemically with various substances called bases, the principal of which are alumina, magnesia, lime, iron, potash, and soda. Although there are many kinds of lava, yet all lavas can be arranged under three great classes according to the quantity of silica they contain. _Acid lavas_ are those in which the quantity of silica is greatest. In these lavas the silica, which varies from 66 to 80%, is combined with small quantities of lime or magnesia, and comparatively large quantities of potash or soda. Some of the most important varieties of acid lavas are known as _trachytes_, _andesites_, _rhyolites_, and _obsidians_. _Basic lavas_ are those containing from 45 to 55% of silica. They are rich in lime and magnesia, but poor in soda or potash. Some of the most important of basic lavas are the _dolerites_ and _basalts_. Generally speaking, basic lavas are of a darker color than acid lavas, and fuse at much lower temperatures. _Intermediate lavas_ are those containing silica in the proportion of from 55 to 66%. While the temperature of liquid lava has not been very accurately determined, yet, since we know that molten lava is able to melt silver or copper, its temperature must be somewhere between 2,500° F. and 3,000° F., the melting point varying with the chemical composition. According to Dana lavas can be divided into the following classes according to their fusibility; i. e., _lavas of easy fusibility_, such as _basalts_; these lavas fuse at about 2,250° F.; _lavas of medium fusibility_, including andesites; these lavas fuse at about 2,520° F.; _lavas of difficult fusibility_, such as trachytes; these lavas fuse at about 2,700° F.