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CHAPTER I.

IGNEOUS ROCKS.

WHEN we consider the position and bedding of these rocks, and the disturbances and other changes they have effected in the strata and beds of other rocks, we cannot doubt that they have been forced upwards from the interior of the earth in a fluid or semi-fluid (viscous) state. They have penetrated and overflowed other formations and then become solid, partly in the clefts and partly on the surface of those rocks. The soft state in which they must have existed during their upheaval was in all probability the result of great heat, in other words, it was a state of igneous fusion; hence the term Igneous Rocks. By process of cooling they then passed over into the solid state, assuming (under different circumstances) a crystallinegranular, a porphyritic, a compact or vitreous texture, sometimes vesicular, or sometimes even a fissile texture (schistose or slaty). Amygdaloids and wackés (as we have already seen) were of later origin, i. e. products of transmutation from original formations.

As regards one great division of these rocks, their former state of fusion is capable of direct proof, and may be observed at the present day; we see them in process of formation from the lava of active volcanoes. These are termed Volcanic Rocks.

In the case of another class of those which we term Igneous Rocks, their former state of fusion is not so clearly evident; indeed we occasionally find their composition, their bedding, or their relative position with other formations in apparent contradiction to their assumed origin. It is supposed that these became solid at a considerable depth, some of them possibly having been poured out in a state of fusion like lava, but in the interior of the earth without reaching the surface, and consequently that their consolidation took place under very high pres

sure, and more slowly than in the case of the volcanic rocks. They are therefore termed Plutonic rocks, and most geologists are agreed on the nature of their origin. The apparent contradictions to an igneous theory of their formation are to be explained by slow cooling, and the changes produced by time and high pressure.

All igneous rocks consist principally of compounds of some kind of felspar (or leucite and nepheline) with pyroxene, hornblende, mica or quartz, generally also with some magnetic iron-ore and other subordinate minerals.

We divide the igneous rocks, whether volcanic or plutonic, into those poor in silica (basic) and those rich in silica (acidic).

The first class, the basic rocks, are distinguished by their deficiency of quartz; by their felspar being generally poor in silica, and frequently richer in lime than that of the acidic rocks, and being mixed with pyroxene or hornblende; by their texture being frequently vesicular or amygdaloidal, very seldom vitreous; and by their generally prevailing dark colour.

The acidic rocks on the other hand are distinguished by a felspar richer in silica; by their frequently containing a large proportion of quartz; by their being rarely vesicular or amygdaloidal, but frequently vitreous; and in general by their lighter colour.

We might add that the basic rocks are more frequently compact and porphyritic than distinctly granular ; more frequently volcanic than plutonic; more frequently found in small unconnected masses than ranging in great tracts or regions; whereas the acidic rocks on the contrary are more frequently distinctly granular and porphyritic than compact; and more frequently extend over vast regions than occur in masses of very circumscribed extent. These data are, however, altogether general in their character, and must be taken with many qualifications.

Bunsen was the first to draw attention to the scientific value of the difference between the basic and acidic rocks, which was previously little known, and had not been carefully investigated. He devoted himself to analysing rock-masses, and from the results of those analyses set up two normal types of composition (see page 364 post). We cannot, however, say that the com

position of the individual rocks of each of Bunsen's groups does more than approximately correspond with those normal values. In fact it would be more accurate to describe the individual values as fluctuating between two extremes than approaching any one central type. With this explanation we present the reader with our view of the composition of the two classes of rocks (the basic and acidic) differing somewhat, but not very greatly, from those of Bunsen.

The principal and most important difference between the two groups is that of the quantity of silica, in which respect there really seems to be a kind of leap with most rocks. The basic rocks in general also contain somewhat more lime and magnesia than the acidic.

Average Compositions of the two classes of Igneous Rocks.

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But the limits which we have above given are sometimes overstepped on each side, and there are igneous rocks which we cannot with mere reference to their chemical composition reckon in either group, and which in fact entirely fill up and annihilate the assumed gap between the two in respect of the content of silica. These rocks of middle character can only be classed with one group or the other by having regard in each case to their geological character or their mineralogical affinities.

If we disregard minor differences, the varieties of igneous rocks are not very numerous; they may be almost reduced to two principal mineral combinations, the other differences consisting chiefly in texture or the presence of accessory or single minerals.

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The two principal combinations are as follows:(1) Felspar poor in silica (in its stead sometimes nepheline or leucite) combined with pyroxene or hornblende, also mica, magnetic iron-ore, and the like.

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(2) Felspar rich in silica, combined with quartz, mica, and occasionally amphibole, and the like.

In presenting this broad view, we do not mean to underrate the importance of the minor differences in the igneous rocks. All those differences are the result of varying conditions and circumstances of their original formation, and are therefore deserving of the greatest attention and study.

We will return to this subject in the concluding chapter, and mention some of the theories in respect to the causes of the various development of the different igneous rocks.

BASIC IGNEOUS ROCKS.

These are compounds of felspar (of various species) with augite, pyroxene, hornblende, or dark coloured mica. They frequently also contain magnetic ironore, sometimes olivine. In some rocks of this class nepheline or leucite takes the place of the felspar. In most, there is an entire absence of quartz.

Their texture is compact, porphyritic, or crystalline; granular, seldom fissile, more frequently vesicular, or amygdaloidal; they are often found in a wackenitic state (wacké).

According to our arrangement as previously indicated, we divide these rocks into two classes expressive of their origin-viz. the volcanic and the plutonic.

1. Volcanic.

These rocks occur in the form of lava at actual volcanoes of the present day; they are also found in districts. where the volcanoes to which they owe their birth have been long extinct. In the latter case they often form isolated conical mountains, or they are found as dykes filling up the rents and fissures of older rocks.

They differ from the plutonic rocks (which have solidified deep down in the earth) by the prevalence of a species of felspar poor in silica, such as labradorite (or in its stead nepheline or leucite); moreover, by the prevalence of augite rather than hornblende; and by the total absence of quartz in their composition. Volcanic rocks also show the traces of their former state of fusion much more distinctly than the plutonic; and they have evidently cooled much more rapidly than them.

All the volcanic rocks hitherto met with are of comparatively recent date, and probably no ancient ones are now existing for the most part they have decayed away.

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