away
from the bases of the mountains toward the sea, and are often
prolonged below the level of the water for a considerable distance
beyond the shore, forming what is commonly known as the continental
shelf or belt of shallows along the coast line. We will now consider
some details concerning the form and structure of mountains.

In almost any mountain region a glance over the surface of the country
will give the reader a clew to the principal factor which has
determined the existence of these elevations. Wherever the bed rocks
are revealed he will recognise the fact that they have been much
disturbed. Almost everywhere the strata are turned at high angles;
often their slopes are steeper than those of house roofs, and not
infrequently they stand in attitudes where they appear vertical. Under
the surface of plains bedded rocks generally retain the nearly
horizontal position in which all such deposits are most likely to be
found. If the observer will attentively study the details of position
of these tilted rocks of mountainous districts, he will in most cases
be able to perceive that the beds have been flexed or folded in the
manner indicated by the diagram. Sometimes, though rarely, the tops of
these foldings or arches have been preserved, so that the nature of
the movement can be clearly discerned. More commonly the upper parts
of the upward-arching strata have been cut off by the action of the
decay-bringing forces--frost, flowing water, or creeping ice in
glaciers--so that only the downward pointing folds which were formed
in the mountain-making are well preserved, and these are almost
invariably hidden within the earth.

[Illustration: Fig. 7.--Section of mountains. Rockbridge and Bath
counties, Va. (from Dana). The numbers indicate the several
formations.]

By walking across any considerable mountain chain, as, for instance,
that of the Alleghanies, it is generally possible to trace a number of
these parallel up-and-down folds of the strata, so that we readily
perceive that the original beds had been packed together into a much
less space than they at first occupied. In some cases we could prove
that the shortening of the line has amounted to a hundred miles or
more--in other words, points on the plain lands on either side of the
mountain range which now exists may have been brought a hundred miles
or so nearer together than they were before the elevations were
produced. The reader can make for himself a conv