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.
But what is, perhaps, most curious about lavas is that when the surface of a freshly broken piece of cold lava is carefully examined, it is found to contain a number of small crystals of such mineral substances as quartz, feldspar, hornblende, mica, magnetite, etc.
The best way to study the different forms of lava crystals is to prepare a thin transparent slice of hardened lava and then examine it with a good magnifying glass. It will be found that the slice consists of a mass of a glass-like material through which the crystals are irregularly distributed, not unlike the raisins and currants in a slice of not over rich plumcake.
When examined by a more powerful glass, such as a microscope, cloudy patches can be seen distributed irregularly through the glass-like mass. When these patches are examined by a higher power of the microscope they are seen to consist of small solid particles of definite forms known as _microliths_ and _crystallites_. It has been shown by a careful study of these minute objects that they form the exceedingly small particles of which crystals are built up.