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Normal faults, as we have seen, have often determined the boundaries between lowlands and highlands. Not infrequently, indeed, it can be shown

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Fig. 70. Section Across Great Fault Bounding The
Southern Uplands.

A, " hard " greywackes, etc.; /, "hard" igneous rocks and overlying conglomerate c.
Demarcation between Uplands and Lowlands not well marked.

that the dominance of certain mountains is due rather to the sinking down of adjacent low-lying tracts than to bulging up of the crust within the mountain-areas

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Fig. 71. Diagram Shction Across Horstgebirge.

a, granite, gneiss, etc., forming the " Horst" ; £, stratified rocks of relatively late age, resting

upon a, dropped down along lines of dislocation //; 0, outlier of b, showing that

the strata b were formerly continuous between A and B.

themselves. Such mountains are, of course, bounded by faults, and are known to German geologists as Horste or Rumpfgcbirgc, the Harz being a good example. The Horste of Middle Europe are composed for the most part of crystalline schists and Palaeozoic rocks, more or less highly flexed and disturbed. The

mountains usually rise somewhat suddenly above the surface of the relatively undisturbed and approximately horizontal Mesozoic strata of the adjacent low grounds, and for a long time it was supposed that these strata in the immediate vicinity of the Horste were littoral deposits. Such, however, is not the case. They are of relatively deep-water origin, and, before faulting supervened, may have covered much of the high lands which now overlook them. It is obvious, in short, that the Horste represent portions of the crust which have maintained their position; they are mountains which testify to a former higher crustal level; the surrounding tracts have broken away from them, and dropped to a lower position.

Probably enough has now been advanced to show that normal faults have had no inconsiderable share in determining surface-features. This, as might have been expected, is most conspicuous in regions of recent crustal deformation and fracture, where epigene action has not had time to effect much modification. In cas.es of very ancient fracture and displacement, however, the surf ace-features, as we have seen, are very greatly modified, and if well-marked disparity of level is still often met with along lines of dislocation, this is mainly due to the fact that rocks of unequal endurance have been brought into juxtaposition. In a case of very considerable displacement it will usually happen, indeed, that crystalline schists, plutonic rocks, or hard Palaeozoic strata will occur upon the high side and relatively softer strata on the low side

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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
—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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