Sift fine sawdust, plaster of Paris, fine coal dust, ground pumice, corn meal, or any other distinctly colored substances in even layers over the stretched rubber dam. Slightly dampen the layers. Releasing the strip, allow the rubber dam to contract very slowly. When it has fully contracted, cut carefully through the layers of material with a thin knife and remove that which is on one side of the cut. The layers will have been folded into irregular undulating folds, thus simulating folded mountains.
Where layers of rock are subjected to slow, uniform and tremendous lateral pressure, they may form undulating folds with little fracturing. (Fig. 119.) The contracting of the interior of the earth, due to cooling,Fig. 119.
has sometimes brought to bear such pressure, and in a few cases undulating folds have been produced.
The best example of this folding is that of the Jura Mountains, between Switzerland and France. Here the almost regular folding of the strata can be seen wherever the streams have cut across the mountains. The mountains are so young that there has been little carving by erosion and the downfolds still form the valleys and the upfolds the ridges.
The rock layers composing these folds contain marine fossils, showing that they were once horizontal and must have been formed in the sea. The longer streams run down the troughs of the folds, but in some places, often where the folds are least high, streams cut across them and pass from one trough to another. Along these transverse stream courses are usually builtthe roads that cross the ridges.
Sometimes the tops of the ridges have been sufficiently worn away or are broad enough to form considerable flat areas, where little villages are situated. But most of the population is found along the longitudinalvalleys, especially where there
FOLDED STRATA.
are cross valleys. Some of the cross streams seem to have no connection with sags in the folds, but appear to have cut their valleys through the folds as fast as they rose, thus indicating that the rate of folding was slow.
198.Massive Mountains. -The mountains already studied areall comparatively low. Probably none of them rises to height exceeding 6000 feet. They are simple in form and outline, and although pleasing features in the landscape, are a bit monotonous. Massive mountains, on the contrary, are varied in form, lofty in height and are among the grandest and most inspiring of Nature"s marvels.
MASSIVE MOUNTAINS.
The high Sierras.
In all ages mountains have been an inspiration to man"s nobler thoughts and higher aspirations. With their heads piercing the azure vault of heaven and towering with gigantic mass above the lower world, they force man to look up, and in the contemplation of theirnobility to forget his meaner self. Like everything else which holds enduring admiration, these are the result of strain and stress and never ceasing battle with the forces of destruction.
The structure of massive mountains is complex in the extreme. Rock layers are often folded, twisted and contorted (Fig. 120) beyond allFig. 120.
recognition of their initial condition. Their uplift has been no simple process, each age has added its peculiar impulse to their growth. As the forces of elevation have been lifting them up, those of degradation have been cutting them down. Their broad brows have been carved into peaks and pinnacles, and gorges and caverns have been cut into their flanks.
The different rock masses which enter into their structure have each assumed its own peculiar lineaments under the carving of the wind, rain, streams, avalanches and glaciers, and thus the variegated beauty of the whole mass has been produced. The central part of massive mountains is composed of igneous rocks, but on the sides overlying these, sedimentary rocks are found. The Rockies, the Alps and the Himalaya Mountains are of this kind.
199.Mountains that no longer Exist. -The mountains which are now such prominent features of the earth"s surface are neither all the same age nor are they the only representatives of this kind of land forms that have ever existed. All the kinds of mountains thus far con- sidered are young in geological age, although some are older than oth- ers.
All parts of the earth"s surface are being gradually worn down by the action of water, but the higher portions are worn more rapidly than those lower, as here the forces of denudation act more intensely. Thus if mountains stop growing, they decrease in height until finally they are too small to be called mountains. Their rocks will be crumpled and folded, and all the characteristics of mountain form will be present except the elevation.
The slant of the rock layers may be such as to indicate a great elevation in former times, but now only the roots of the mountains are left and the region is of very moderate elevation. Regions of this kind are found in many parts of the earth.
In the Appalachian highlands of Pennsylvania the rocks show that they were once folded into ridges and troughs something like those of the Jura. But now the arches have been worn away, andthe existing ridges are due to the
resistance which the harder layers
BEN NEVIS.
A mountain much worn down but still high.
offer to erosion. These ridges are as likely to occur where formerly the troughs of the folds were as in what were the crests.
The configuration of the country is not at all as it was when the rocks were folded. The elevation then was much greater than the highest ridges at present. If the beds should be reconstructed as they now lie, they would indicate a height much greater than the mountains ever had at any time. Research shows that these mountains have been lifted up and worn down more than once.
THE MAT TERHORN.
