CHAPTER XIV.

ERUPTIVE ROCKS AND MINERAL VEINS IN THE ARCHITECTURE OF THE EARTH'S CRUST.

Not only have sedimentary formations since their deposition been hardened, plicated, fractured, and sometimes even turned into crystalline schists, but into the rents opened in them new masses of mineral matter have been introduced which, in many regions, have entirely changed the structure of the crust below and the appearance of the surface above. Broadly speaking, there are two ways in which these new masses have been wedged into their places. First of all, eruptive material in a molten, or at least in a viscous or plastic condition, has been thrust upward into the cool and consolidated crust of the earth; and in the next place, various ores and minerals have been deposited from solution in cracks and fissures, which they have entirely filled up. To each of these two kinds of later rocks attention will be given in this chapter.

Eruptive Rocks.

The rise of eruptive matter thrust upwards from lower depths within the planet is one of the causes by which the structure of the crust has been most seriously affected. In chapter ix. reference was made to some of the features connected with the protrusion of molten rocks in the production of volcanoes, and more particularly to those subterranean changes which, when all the outer and ordinary tokens of a volcano have been swept away, remain as evidence of former volcanic action, even in districts where every symptom of volcanic activity has long vanished. We have now to inquire, generally, in what forms eruptive matter has been built into the earth's crust, and what changes it has produced there, apart from those superficial manifestations which are the visible signs of volcanic action.

When a mass of lava or other fluid or viscous material is forced upwards from the heated interior of the earth towards the surface, the form which it finally takes and in which it cools and solidifies depends upon the shape of the rent or cavity into which it has been thrust. We may compare such a mass to a quantity of melted iron escaping from a blast-furnace. The shape taken by the iron will, of course, be fixed by that of the mould into which it is allowed to run. The crust of the earth, as was pointed out in the previous chapter, has undergone extensive movements, whereby its rocks have been crumpled and broken. It consequently presents in different parts very different degrees of resistance to any force acting upon it from below. Where much shattered it yields to the upward pressure of eruptive materials, and these force a way along its fissures or between the beds and joints of the strata. According to the form of the mould in which they have solidified, we may classify the eruptive rocks of the crust into (1) bosses; (2) sheets; (3) veins and dykes; and (4) necks.

BOSSES.-These are circular, elliptical, or irregularly shaped eruptive masses which, while still in a liquid or viscous state, have been ejected into irregular rents of the earth's crust and have solidified there. They consist of various crystalline rocks, more especially granite, syenite, quartz-porphyry, diorite, diabase, and basalt-rocks, and vary in width from a few yards to several miles. Being generally harder than the surrounding rocks, they commonly stand up as prominent knobs, hills, or ridges. Their presence at the surface, however, is due, not to their original protrusion there as in a

FIG. 105. Outline and section of a Boss (a) traversing stratified rocks (bb).

volcanic cone, but to the removal of the overlying part of the original crust under which they cooled and consolidated. Every boss is thus a witness of the extensive wearing away of the surface of the land (Fig. 105).

In some large bosses, there may have been a complex system of fissures into which the eruptive material rose. Forced upwards into these, the molten rock would no doubt envelope separated masses of the crust, and might bear them along with it in its ascent. We may even conceive it to have melted down such enveloped masses. Pushing the rocks aside and thrusting itself into every available crack in them, the eruptive mass would work its way across the crust. Where it succeeded in opening a passage to the surface, ordinary volcanic phenomena would take place, such as disruption of the ground, ejection of stones and ashes, and outflow of lava. But, no doubt, in a vast number of cases, no such communication was ever effected. The eruptive material paused in its upward passage and consolidated below ground.

No rock affords more interesting bosses than granite. Two features are there especially well developed-the marginal veins or dykes and the surrounding ring of metamorphism produced in the rocks through which the granite has risen. Granite has invaded many different kinds of rocks, and has effected various kinds of change in them. Round its margin, large numbers of veins or dykes of granite or quartz-porphyry often strike out from it into the surrounding rocks. There can be no doubt that these are portions of the granite material, squeezed into cracks that opened in the crust around it during its ascent. More important is the change that can be observed to have taken place in the rocks immediately surrounding the boss. The granite at the time of its protrusion was probably in a molten or pasty condition and impregnated with hot water or steam and vapours. For a distance varying up to two or three miles, the rocks next to it have undergone alteration, the nature and amount of which appear to have been in great measure dependent on their chemical and mineralogical composition. This metamorphism consists partly in mere induration, but still more in the development of new minerals, or a new crystalline structure, even out of non-crystalline sedimentary materials. The very same rock, which is elsewhere a dark limestone full of shells, corals, or other organic remains, becomes a white crystalline marble next the granite, with no trace of any organisms, and so unlike its usual condition that no one would readily believe it to be the same rock. Again, a dark shaly sandstone or greywacke traced towards the granite begins to show an increasing amount of mica. New minerals likewise make their appearance, particularly garnets, until the rock entirely loses its sedi

FIG. 106. Ground-plan of Granite-boss with ring of Contact-Metamorphism; (a) sandstones, shales, etc., dipping at high angles in the direction of the arrows; (6) zone or ring within which these rocks are metamorphosed; (c) granite sending out veins into b.

mentary structure and becomes a garnetiferous mica-schist. Shales and slates, as they approach the granite, likewise present a remarkable development of fine mica-plates, and pass into argillaceous schists or phyllites, with crystals of chiastolite or other minerals developed in them. The alteration of rocks round eruptive masses is called contact-metamorphism.

What the cause may be of this remarkable alteration has not yet been satisfactorily made out. In some bosses, the mere heat of the eruptive material was probably sufficient to produce change. There must often have been also a copious discharge of hot vapours and water, which would