CHEMICAL COMPOSITION OF LAVAS.

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eight elements are present in very unequal proportions. Oxygen makes up nearly one-half the weight of all lavas. Almost all the other elements found in lavas exist in combination with oxygen, so that lavas consist entirely of what chemists call 'oxides.' This is a most remarkable circumstance, which, as we shall presently see, is of great significance. The metalloid silicon makes up about one-fourth of the weight of most lavas, and the metal aluminium about one-tenth. The other five elements vary greatly in their relative proportions in different lavas.

In all lavas the substance which forms the greatest part of the mass is the compound of oxygen and silicon, known as silica or silicic acid. In its pure form, this substance is familiar to us as quartz, or rock-crystal and flint. Silica is present in all lavas in proportions which vary from one-half to four-fifths of the whole mass. Now, this substance, silica, has the property of forming more complex compounds by uniting with the other oxides present in lavas-namely, the oxides of aluminium, magnesium, calcium, iron, potassium, and sodium. Silica is called by chemists an acid, the other oxides in lavas are termed bases, and the compounds of silica with the bases are known as silicates. Hence we see that lavas are composed of a number of different silicates the silicates of aluminium, magnesium, calcium, iron, potassium, and sodium.

The above statements will perhaps be made clearer by the accompanying table from which it will be seen

Oxygen

that lavas are compounds in varying proportions of six kinds of salts-namely, the silicates of alumina, magnesia, lime, iron, potash, and soda.

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

DIFFERENT KINDS OF LAVA.

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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 acidlavas, 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.

The basic lavas are usually of much darker colour than the acid 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.

E

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.

Before we part from this subject of the classification of lavas according to their chemical composition, it will be well to point out that there exists a small group of lavas which stand quite by themselves, and cannot be referred to either of the classes we have indicated. They contain a smaller proportion of silica, and a much larger proportion of magnesia and oxide of iron than the other lavas, and may be made to constitute a small sub-group, to which we may apply the term of ultrabasic lavas.' Although much less widely distributed than the other varieties, they are, in some respects, as we shall presently have to point out, of far greater interest to the geologist than all the other kinds of lavas.

We will now proceed to consider the facts which are brought to light concerning the nature of lavas, when they are studied by the aid of the microscope. Although most lavas appear at first sight to be opaque substances, yet it is easy to prepare slices of them which are sufficiently thin to transmit light. In such thin transparent slices we are able to make out, by the

MINUTE STRUCTURE OF LAVAS.

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aid of the microscope, certain very interesting details of structure, which afford new and important evidence bearing on the mode of origin of these rocks.

Most lavas are capable of being melted by the heat of our furnaces; but the different kinds of lava vary greatly in the degree of their fusibility. The basic lavas, or those with the smallest proportion of silica, are usually much more easily fusible than those which contain a high percentage of silica, the acid lavas.

Now, it is a very noteworthy circumstance, that when a lava is artificially fused it assumes on cooling very different physical characters to those which were presented by the original rock.

If we examine the freshly-broken surface of a piece of lava, we shall, in most cases, find that it contains a great number of those regular-shaped bodies which we call crystals; in some cases these crystals are so small as to be scarcely visible to the naked eye, in others they may be an inch or more in length. Most lavas are thus seen to be largely made up of crystals of different minerals. The minerals which are usually contained in lavas are quartz, the various kinds of felspar, augite, hornblende, the different kinds of mica, olivine, and magnetite.

But when a piece of lava is melted in a furnace, all these crystalline minerals disappear, and the resulting product is the homogeneous substance which we call glass. If, as many suppose, lavas acquire the fluidity which they possess when issuing from volcanic vents