The Elements of Geology eBook

This eBook from the Gutenberg Project consists of approximately 384 pages of information about The Elements of Geology.

The Elements of Geology eBook

This eBook from the Gutenberg Project consists of approximately 384 pages of information about The Elements of Geology.

A magnificent system of dikes radiates from the Peaks to a distance of fifteen miles, some now being left by long erosion as walls a hundred feet in height (Fig. 239).  Intrusive sheets fed by the dikes penetrate the surrounding strata, and their edges are cut by canyons as much as twenty-five miles from the mountain.  In these strata are valuable beds of lignite, an imperfect coal, which the heat of dikes and sheets has changed to coke.

LACCOLITHS.  The laccolith (Greek laccos, cistern; lithos, stone) is a variety of intrusive masses in which molten rock has spread between the strata, and, lifting the strata above it to a dome-shaped form, has collected beneath them in a lens-shaped body with a flat base.

The Henry mountains, a small group of detached peaks in southern Utah, rise from a plateau of horizontal rocks.  Some of the peaks are carved wholly in separate domelike uplifts of the strata of the plateau.  In others, as Mount Hillers, the largest of the group, there is exposed on the summit a core of igneous rock from which the sedimentary rocks of the flanks dip steeply outward in all directions.  In still others erosion has stripped off the covering strata and has laid bare the core to its base; and its shape is here seen to be that of a plano-convex lens or a baker’s bun, its flat base resting on the undisturbed bedded rocks beneath.  The structure of Mount Hillers is shown in Figure 248.  The nucleus of igneous rock is four miles in diameter and more than a mile in depth.

Regional intrusions.  These vast bodies of igneous rock, which may reach hundreds of miles in diameter, differ little from bosses except in their immense bulk.  Like bosses, regional intrusions give off dikes and sheets and greatly change the rocks about them by their heat.  They are now exposed to view only because of the profound denudation which has removed the upheaved dome of rocks beneath which they slowly cooled.  Such intrusions are accompanied —­whether as cause or as effect is still hardly known—­by deformations, and their masses of igneous rock are thus found as the core of many great mountain ranges.  The granitic masses of which the Bitter Root Mountains and the Sierra Nevadas have been largely carved are each more than three hundred miles in length.  Immense regional intrusions, the cores of once lofty mountain ranges, are found upon the Laurentian peneplain.

Physiographic effects of intrusive masses.  We have already seen examples of the topographic effects of intrusive masses in Mount Hillers, the Spanish Peaks, and in the great mountain ranges mentioned in the paragraph on regional intrusions, although in the latter instances these effects are entangled with the effects of other processes.  Masses of igneous rock cannot be intruded within the crust without an accompanying deformation on a scale corresponding to the bulk of the intruded mass. 

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The Elements of Geology from Project Gutenberg. Public domain.