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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;
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.) appearing in the Carboniferous, some in the JuraTrias, and others in the Cretaceous. From the lowest to the highest laccolith the range is not less than 4000 feet, those which are above not infrequentlyoverlapping those which lie below. "Their horizontal distribution is as irregular as the arrangement of volcanic vents. They occur in clusters, and each cluster is marked by a mountain. In Mount Ellen there are perhaps thirty laccolites; in Mount Holmes there are two ; and in Mount Ellsworth one. Mount Pennell and Mount Hillers have each one large and several small ones." The highest of these mountains attains an elevation of over 11,000 feet, rising some 5000 feet above the plateau at its base. The strata of which that plateau is built up are approximately horizontal, and appear at one time to have been covered by some thousands of feet of Tertiary deposits, the nearest remains of which occur at a distance of thirty miles from the Henry Mountains. Mr. Gilbert is of opinion that the laccolites were most probably intruded after the deposition of the Tertiary strata, and before their subsequent removal by erosion.
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
Fig. 74. Diagrammatic Section Of A Laccolith Showing Dome-shaped
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
The whole structure of the Henry Mountains shows that the actual surface was affected by those intrusions, the horizontal strata being arched upwards so as to form dome-shaped elevations, rising prominently above the general level of the plateau. The laccoliths are all of considerable size, the smallest measuring more than half a mile, and the largest about four miles in diameter. The mountains formed by them consist of a group of five individuals separated by low passes, but having no definite range or trend. The subsequent erosion of these mountains, Mr. Gilbert remarks, has given the utmost variety of exposure to the laccoliths. In some places these are not yet uncovered, and we see only the arching strata which overlie them, the strata being cut across by only a few dikes or traversed by a network of dikes and sheets. In other places denudation has partly bared the laccoliths or even completely exposed them, so that their original form can be seen. In yet other places the bared laccolith itself has been attacked by the elements, and its original form more or less changed. It is even quite possible that occasionally laccoliths may have been entirely demolished, and that some of the truncated dikes now visible at the surface may mark the old fissures or conduits through which such vanished laccoliths were injected.
From the evidence just referred to, it is obvious that intrusions of igneous rock, if of sufficient thickness, are capable of warping the surface, and of forming more or less considerable elevations. But as erosion tends to reduce all such upheavals more or less rapidly, it is only those of relatively recent age that can retain any trace of their original configuration. All masses of intrusive rock of great geological antiquity, which now form hills and mountains, do so in virtue of their greater resistance to the action of epigene agents. They may have arched up the rocks underneath which they formerly lay buried, and so