(2) When an area is elevated above the sea the natural forces of erosion, wind, water, changes of temperature. etc., begin their attack upon the land, and slowly cut it down, the waters collecting in rills, brooks, and rivers, cut ting small and large valleys with a variety of forms and distribution due to the varying hard ness and the structure of the materials cut through. If carried far enough this process will ultimately bring the elevated area to sea level. At certain stages in the process certain areas, because of their hardness or structure, may stand high above the surrounding areas which have been worn away, and may be called mountains. In so far as mountains have already been formed by folding and elevation, subsequent erosion only modifies their shapes, but where uplift has not left the land in mountain form subsequent de nudation may bring it. to this point. Mountains formed or modified in this way have a variety of shapes. When the strata are horizontal a hard layer at the top may resist erosion sufficiently long to make the mountain a flat or table top mountain. Hard volcanic material about a vol cano may resist erosion more than the surround ing material, and the area thus stand as a lava topped mountain. If the strata are munch folded
erosion cuts down along lines of least resist ance. usually following the softer strata, leaving the edges of the harder ones as ridges. This state of affairs appears in the Appalachian system, a system owing its present features to and elevation combined with differential denudation along softer layers.
(3) Mountains formed by eruption of igneous rocks are of common occurrence. Volcanic moun tains formed by lavas occurring either singly or grouped in lines are well known. Vesuvius, Rainier. Hood. Saint Helen's. and Shasta :ire ex amples of these. The volcanic materials may be so grouped as to form an upland of consider able extent, as the Cascade range. Eruptions of igneous rocks which never reach the surfac.e also form mountains by bulging up the strata above them. upward toward the surface they find it easier at a certain point to spread out in a globular form (called laccolite), arch ing strata above them, than to break through the overlying strata and come to the surface. The mountain thus formed is a dome with a nucleus of igneous material. The Henry and Huerfann mountains of the Western United States are of this type.