Now, in some lavas the acid constituent, or silica, is present in much larger proportions than in others. Those lavas with a large proportion of silica are called 'acid lavas,' those with a lower percentage of silica, and therefore a higher proportion of the bases, are known as the 'basic lavas.' It is convenient to employ the term 'intermediate lavas' for those in which the proportion of silica is lower than in the acid lavas, and the proportion of the bases is lower than in the basic lavas. The acid lavas contain from 66 to 80 per cent, of silica; they are poor in lime, magnesia, and oxide of iron, but rich in potash and soda. The basic lavas contain from 45 to 55 per cent, of silica; they are rich in magnesia, lime, and oxide of iron, but poor in soda and potash. In the intermediate lavas the proportion of silica varies from 55 to 66 per cent. As the basic-lavas contain a larger proportion of oxide of iron and other heavy oxides than the acid-lavas, the former have usually a higher specific gravity than the latter; it is, indeed, possible in most cases to distinguish between these different varieties by simply weighing them in water and in air. [Sidenote: DIFFERENT KINDS OF LAVA.] The basic lavas are usually of much darker colour than the add lavas--the terms acid lavas, intermediate lavas, and basic lavas correspond indeed pretty closely with the names trachytes, greystones and basalt, which were given to the varieties of lavas by the older writers on volcanoes, at a time when their chemical constitution had not been accurately studied. Fresh lavas of acid composition are usually nearly white in colour, intermediate lavas are of various tints of grey, and basic lavas nearly black. It must be remembered, however, that colour is one of the least persistent, and therefore one of the least valuable, characters by means of which rocks can be discriminated, and also that by exposure to the influence of the atmospheric moisture the iron present in all lavas is affected, and the lavas belonging to all classes, when weathered, assume reddish and reddish-brown tints. Geologists have devised a great number of names for the various kinds of lava which have been found occurring round volcanic vents in different parts of the world, and the study of these varieties is full of interest. For our present purpose, however, it will be sufficient to state that they nearly all fall into five great groups, known as the Rhyolites, the Trachytes, the Andesites, the Phonolites, and the Basalts. The Rhyolites are acid lavas, the Basalts are basic lavas, and the Trachytes, Andesites, and Phonolites, different kinds of intermediate lavas, distinguished by the particular minerals which they contain.