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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
—VOLCANOES, STRUCTURE AND FORM OF—MUD-CONES—GEY-
SERS—FISSURE-ERUPTIONS — VOLCANIC PLATEAUX — DENUD-
ATION OF VOLCANOES, ETC., AND RESULTING FEATURES.

IN preceding pages we have had frequent occasion
to refer to igneous rocks. These, as we have
seen, may be broadly grouped under two heads—Plu-
tonic 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 rela-
tively great depths, while the term volcanic includes

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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

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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;

[graphic]

Fig. 72. Mountain Of Granite.

g, granite sending veins into schists, etc., CO. 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.)

[graphic]

F10. 73. Plain Of Granite Overlooeed By Mountains Of Schists, Etc.

/, granite; *, schists. «e. 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

marlcable examples known are the laccoliths or laccolites (stone cisterns) of the Henry Mountains (southern Utah), which have been described by Mr. Gilbert. In that region molten rock, instead of ascending to the surface and building up mountains by successive eruptions, has stopped at a lower horizon, insinuated itself between the strata, and opened for itself a chamber by lifting all the superior beds. (See Fig. 74.) Proceeding from a laccolith are in

[graphic]

Fig. 74. Diagrammatic Section Of A Laccolith Showing Dome-shaped
Elevation Of Surface Above The Intrusive Rock. (After G. K. Gilbert.)

P, pipe or conduit; xA, sheet; d dy dikes.

trusions of the same kind of igneous rock (trachyte), some of which (sheets) have squeezed themselves between adjacent beds, while others (dikes) traverse the strata at less or greater angles. These remarkable rocks have been intruded in a great series of strata ranging in age from Carboniferous to Cretaceous, amongst which they are irregularly distributed, some

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