It is not necessary in all cases to assume that the block mountains of a faulted district represent the blocks which in the adjustments were left the highest. Erosion in the course of time accomplishes marvels of transformation, and it may result that heavy masses of more resistant rock eventually project the highest, even though they may represent the downthrown blocks in the fault mosaic ([Fig. 43], [p. 60]).
Fig. 478.—Tilted crust blocks in the Queantoweap valley.
Where in addition to undergoing changes of level the earth blocks have been tilted, the features long since described from our western interior basin as “Basin Range structure” are developed. Here the upper surface of the disturbed earth blocks betrays the evidence of a definite tilt in some one direction ([Fig. 478], and [Fig. 431], [p. 402]).
Mountains of outflow or upheap.—An important type of mountain, generally described as volcanic, may be due either to the outflow of lava at the earth’s surface, or to accumulations of separated fragments of lava, first thrown into the air, and then deposited by gravity or admixed with water as volcanic mud. Such mountains, both before and after modification by erosion, assume the strikingly characteristic forms which have been fully discussed in Chapters IX and X. The dominant types are the lava dome and the puy, the cinder cone, and the more complex composite cone. Excepting only the surface produced by the few great fissure eruptions and the semivolcanic mesa type, the individual mountains of volcanic origin develop features with notably circular bases.
Fig. 479.—Pen drawing of the laccolite of the Carriso Mountain by W. H. Holmes, which shows the jagged surface of the igneous rock core and the sloping tables which still remain of the roof of sedimentary rocks (after Cross).
Fig. 480.—Map of laccolitic mountains. A portion of the Judith Mountains, Montana. The intrusive igneous rock is shown in black (after Weed).
Domed mountains of uplift—laccolites.—At a considerable number of widely separated localities upon the earth’s surface, mountainous regions are encountered, the central areas or cores of which are composed of intrusive igneous rock such as granite, and about this core the sediments dip away in all directions as though they had once formed a continuous roof above it and had been forced into this dome by hydrostatic pressure of the once viscous material beneath ([Fig. 152], [p. 143], and [Figs. 479] and [480]). Examples of such domed mountains of uplift were first described by Gilbert from the Henry Mountains of Utah, but instances are furnished by many elevated tracts, especially within the western United States. Such mountains are known as laccolites, but when one margin at least of the igneous core corresponds to a displacement, the mountain is described as a bysmalite ([Fig. 481]).