It is in all

neither will it reveal the actual proportions of the different minerals or elements present in stratified rocks. It is only by chemical analysis that we can ascertain the ultimate chemical composition of such rocks, and the relative proportion of the different elements they contain. cases desirable to ascertain the chemical composition of a rock, for it often reveals the presence of minute quantities of certain minerals or elements which cannot be discovered by the miscroscope: and in the case of a glassy igneous rock we can compare the analysis with that of allied crystalline varieties, the mineral constituents of which are already known.

CHAPTER III.

ORIGINAL OR IGNEOUS ROCKS.

A. Unaltered.

As these have solidified from a state of fusion by heat, their present appearance will obviously be the result of two causes: 1, their chemical constitution; 2, the circumstances of cooling. To the former is due the presence of certain minerals and the absence of others; on the latter depends the structural condition of the rock, whether it is a glass, or has assumed a more or less crystalline structure. There are in fact five points of difference among Igneous rocks, which are of special importance; these are:

1. Mode of occurrence as rocks.

2. Internal structure.

3. Chemical composition.

4. Mineral composition.

5. Specific gravity.

Each of these characters might be made the basis of a classification.

1. Mode of Occurrence.-Viewed as rock-masses, they occur in various positions and exhibit different forms, according to the circumstances under which they have solidified. These differences will be described in a future chapter; but as it was formerly customary to classify Igneous rocks according to such differences, they may be briefly referred to here. The divisions made on this basis were: (1) VOLCANIC, or those which had cooled down in the open air; (2) TRAPPEAN, or those which had solidified under comparatively slight pressure, as on the bed of the

ocean; and (3) PLUTONIC, or those which had solidified deep within the earth. The name given to the second group was from a Swedish word, trappa, a stair, because the ends of the tabular rock-masses referred to this division had often a step-like aspect. More careful investigation, however, showed that many of the more typical members of this division had solidified in the open air, and that it was impossible to fix on any set of characters which separated them from the ordinary volcanic rocks or lavas, as they are often called. It then became usual to divide the igneous rocks simply into VOLCANIC and PLUTONIC, but here it at once became difficult, owing to the more ready crystallization of the basic kinds, to know where to draw the line. Hence these terms have been abandoned for purposes of precise classification, and are only used as general inclusive names of no very definite meaning, which, however, are often useful.

2. Structure.-If the student will examine any masses of igneous rock in situ, or any collection of rock specimens, he will soon perceive that they exhibit great differences of texture, some being glassy, some compact and stony, others plainly crystalline.

We know also, as a matter of experiment, that from the same materials, after fusion, may be obtained either a translucent glass or an opaque stone, built up more or less completely of crystalline particles, and by means of the microscope it is found that the matrix, or ground mass, of a rock may be in one of the following conditions :

:

A. It may be a true glass (when it is often termed the base). In this case, when a thin slice of it is placed between the crossed nicol prisms of a microscope with a polarizing apparatus, the field remains dark as the stage is rotated. Rocks with a glassy base are said to

be vitreous.

B. If small, faint, and rather ill-defined patches of light, like the ghosts of crystals, appear and disappear as the stage is rotated, so that no part remains dark throughout a whole revolution, the ground mass is said to be cryptocrystalline.

c. If the patches are more definite in shape and rather larger in size, sometimes showing bright tints, so that it is

Ꮓ

possible to identify the different component minerals, the mass is said to be micro-crystalline. In this state, however, the crystalline grains are often a little indefinite at the boundaries, and do not commonly exhibit the outline of their normal form.

D. If the slide is a mass of well-defined crystals, clearly distinguishable one from another, of which those first solidified generally give indications of their normal external form, the rock is then said to be crystalline.

The second of these conditions is in many cases the result of devitrification, the gradual change from a vitreous to a semi-crystalline state which takes place in a glass under certain conditions. In such case a crypto-crystalline rock might fairly be regarded as metamorphosed; but seeing that we are not yet able to distinguish satifactorily between cases where the structure is original and where it is secondary, the separation is not generally attempted. Neither is it easy at present to separate rocks with this structure from those of the micro-crystalline group, so that we shall speak of both inclusively as semi-crystalline.

Glassy and semi-crystalline varieties are of frequent occurrence among the acid rocks, but are rare and more limited in mass among the basic rocks, and in some instances have not yet received distinctive names.

Ophitic Structure. This is caused by the occurrence of large crystals of one mineral which include a number of smaller and previously formed crystals of another mineral. Thus in many rocks consisting of felspar and augite, the former has crystallized out before the latter, so that the augite seems to form a ground mass or matrix in which the felspar crystals are embedded. Such rocks were formerly called Ophites, but the same kind of structure has since been found in many other rocks, so that the name is only used as a descriptive one. Such rocks often present the appearance known as lustre-mottling, the small crystals interfering with the play of the light from the broken surfaces of the large crystals as the specimen is turned in the hand.

Porphyritic Structure.-Conspicuous isolated crystals of any one or more of the component minerals may occur in glassy, semi-crystalline, or crystalline rocks, giving what is

termed a porphyritic structure. The name Porphyry was first applied to the dark red or purple rock, speckled with light-coloured felspar, obtained from Jebel Dokhan in Egypt, and largely used by the Romans for decorative purposes. By some means the term became applied in process of time to rocks of the most diverse mineral composition, but exhibiting the same peculiarity of structure, viz., the conspicuous occurrence of one mineral in a more compact matrix. The adjective porphyritic is now always used with this meaning, which of course the etymology (porphyra, purple) does not sanction, and the name Porphyrite has been applied to a definite kind of rock; but the term Porphyry is seldom used as a designation in England.

Crystalline rocks present several textural varieties: they may be coarsely crystalline, like many Granites; they may be finely crystalline and even grained (granulitic); they may be porphyritic; and finally, two of the component minerals may be so intermixed and interwoven with one another by simultaneous crystallization as to produce a curious and easily recognized pattern. This is the structure of Pegmatite or Graphic Granite and hence it is termed pegmatitic or graphic. Sometimes the matrix of a porphyritic rock has this structure on a minute scale, which is often called micro-pegmatitic, but the term micrographic, recently proposed by Professor Bonney, is preferable. A rock with a glassy base is said to be microlithic when it is crowded with minute crystals or microliths.

If the mass of a rock is full of small cavities or vesicles it is said to be vesicular; the cavities are often elongated in one direction, and when they are subsequently filled up with some mineral, such as quartz or calcite, the infiltration is called an amygdule or amygdaloid (amygdalon, an almond), and the rock is said to be amygdaloidal.

3. Chemical Composition.-All the minerals which ordinarily enter into the composition of igneous rocks are silicates, and free silica in the form of quartz is also frequently present. The total percentage of silica which they possess is sometimes as high as 80, sometimes as low as 40, or a little less; hence these rocks are often simply divided into acid and basic, sccording as they contain more or less than 60 per cent. of this mineral.