of the fault. However prolonged and intense epigene action may have been, such a fault will nevertheless cause a marked feature at the surface, so long as the general surface of the land remains considerably above the base-level. But when the latter is approached denudation will eventually cease on the low side of the fault, while material will continue to be removed from the high side, and the disparity between the two will thus tend gradually to disappear. In short, the irregularities of surface determined by the presence of faults pass through the same cycle of changes as all other kinds of geological structure. Should the base-level remain undisturbed epigene action must eventually reduce every inequality, no matter what its origin may have been. Again, were such a reduced land-surface to be re-elevated and converted into a plateau, the lines of dislocation that happened to separate areas of hard rock from regions of soft rock would once more determine the boundaries between high and low ground. The surface of the soft rocks

would be lowered most readily, while the more durable hard rocks would come to form elevations. In a word, the features that obtained before the land was reduced to base-level would, under the influence of denudation, tend to re-appear.

CHAPTER VIII

LAND-FORMS DUE DIRECTLY OR INDIRECTLY TO IGNEOUS ACTION

PLUTONIC AND VOLCANIC ROCKS-DEFORMATION OF SURFACE CAUSED BY INTRUSIONS-LACCOLITHS OF HENRY MOUNTAINS

IN

-VOLCANOES, STRUCTURE AND FORM OF-MUD-CONES-GEY-
VOLCANIC PLATEAUX

SERS FISSURE-ERUPTIONS

--- DENUD.

ATION OF VOLCANOES, ETC., AND RESULTING FEATURES.

N preceding pages we have had frequent occasion to refer to igneous rocks. These, as we have seen, may be broadly grouped under two heads-Plutonic rocks and Volcanic rocks. The former have cooled and solidified at a less or greater depth below the surface; the latter, on the other hand, have been extruded at or near the surface. No hard and fast line, however, can be drawn between these two groups. All plutonic rocks are indeed intrusive-they have solidified below ground; but the same is true of the sheets and dikes which traverse a volcano, and which, along with the bedded lavas and tuffs they traverse, are properly described as of volcanic origin. It will be understood, then, that the term plutonic is restricted to intrusive rocks which have consolidated at relatively great depths, while the term volcanic includes

all igneous rocks which enter or have entered into the formation of a volcano, or which have evidently proceeded from any focus or foci of eruption.

It is needless to say that we can know nothing by direct observation of the conditions and phenomena which attend the intrusion of deep-seated plutonic rocks. But so many of these have been laid bare by denudation, their composition and their relation to surrounding rock-masses have been so carefully studied, that geologists have learned much concerning igneous action of which but for denudation they must have remained largely ignorant. They have ascertained, for example, that such lavas as rhyolite, andesite, and basalt have their deep-seated equivalents in the plutonic granites, syenites, and gabbros. That is to say, we know that the same molten mass solidifies at great depths as granite or other wholly crystalline rock, and at the surface as rhyolite or other semi-crystalline lava. In short, plutonic rocks and their volcanic equivalents have practically the same chemical composition. An acid lava comes from an acid magma, a basic lava from a basic magma. Hence it is inferred that many plutonic rocks now exposed by denudation may have been the deep-seated sources from which ancient lavas have proceeded. On the other hand, there is reason to believe that many plutonic masses may never have had any such volcanic connections.

But whether or no a given plutonic mass be the deep-seated source of some long-vanished volcano or volcanoes does not concern us here. We have sim

ply to recognise the fact that its exposure at the surface is the direct result of profound denudation. Whether its intrusion had any effect in deforming the surface we cannot tell. Probably, in cases where none of the material was extruded to the surface by contemporaneous volcanic action, there may have been some bulging up of the ground. Deformation of the crust, in short, may quite well have accompanied the subterranean movements of great masses of molten matter. But so long a time has elapsed since the granites and other highly crystalline plutonic rocks were intruded-so enormous has been the thickness of rock removed from above them-that such intrusion cannot be said to have had any direct effect in the production of existing surface-features. It is quite true that many hills and mountains are composed largely or even exclusively of plutonic rocks;

g, granite sending veins into schists, etc., (s). The schists have been more readily lowered by erosion than the granite.

but that is simply owing to the fact that these rocks are usually more durable than the rocks through which they rise. When, as not infrequently happens, plutonic masses are of less durable consistency and

construction than the rocks that surround them, the latter invariably dominate and overlook the former. Thus while granite often forms prominent mountains (Fig. 72, p. 175), not infrequently it is found occupying low tracts flanked by mountains of schist, slate, or other rock. (Fig. 73.)

FIG. 73.

9

PLAIN OF GRANITE OVERLOOKED BY MOUNTAINS OF SCHISTS, ETC. g, granite; s, schists, etc. The granite has been more readily lowered by erosion than the surrounding schists.

We must conclude, then, that whatever effect may have been produced at the surface by the intrusion of the more ancient plutonic rocks of England and other countries, such superficial effects, if any, have long since disappeared. The present configuration of the ground occupied by such rocks is wholly the result of epigene action. But when we consider the phenomena of more recent intrusions of igneous rock, we find reason to conclude that these have not only had a direct effect at the surface, but that this effect has not yet in all cases been removed by denudation. The ground has bulged up, and the swelling of the surface is still conspicuous. Among the most re