This is seen in the region of the Grand Ca？on of the Colorado， wherethe Kaibab Plateau stands about 2000 feet above the Colorado Plateauand steep cliffs bound it on both its east and west sides. These fault cliffs， as they are called， are found at several other places in this region， showing that the whole area was much broken when it was uplifted. The Kaibab Plateau itself is so much higher than the plateaus on either side that it intercepts sufficient rainfall to support forests， whereas the plateaus about it are almost barren of trees.
In the walls of the Colorado Ca？on some of these great breakage lines can be traced and the same strata seen to be thousands of feet higher on one side of the line than on the other. In front of these breakage cliffs or fault cliffs， accumulations of débris extend along the entire distance， showing that since the uplift there has been time for much erosion even in this dry region. TheColorado River passes over these great
A FAULT.
faults regardless of their existence. The ca？ons in the region seem not to have been influenced by the faulting. Probably it took place too slowly.
194.Hills and Mountains. -Irregular elevations of the earth"ssurface are called hills， or mountains when they are of considerable height. In the general use of these terms there is no exact line of sepa- ration. Elevations which in mountain regions would be called hills would in a flat region be called mountains. As a rule， elevations are not termed mountains unless they are at least 2000 feet high. But if the general elevation of the country is great， as in the lofty regions of the Rockies， an elevation to be termed a mountain must rise to a striking height above the generally elevated surface， which is itself nearly ev- erywhere more than 4000 feet above the sea.
195.Structure of Mountains. -Mountains are the results of de- formations in the earth"s crust， due to causes not fully understood and the study of which is a part of geology. The crust of the earth has beenAPPALACHIAN PLATEAU.
A range of old mountains greatly reduced in height.
folded， pushed up， crumpled and in many ways distorted so that some portions have been elevated to a considerable height above sea level. Where these elevated portions have not remained long enough to be worn down， they form mountains.
All lofty mountains have been elevated in comparatively recent geological time， but this of course means millions of years ago. If mountains now lofty were geologically old， they would long ago have been worn down. The older mountains of the earth are all comparatively low， not because they were never elevated as high as the lofty mountains of to-day， but because their greater age has longer subjected them to erosion and thus reduced their height.
It is difficult to classify the different kinds of mountains， for very few of them are simple in their structure， but certain kinds of mountain forms are easily distinguished.
196.Block Mountains.
Experiment 131. -Take three pieces of smooth， straight-edged boards， two of which are about 15 × 55 cm. on a side and the other 8× 55. Place theseflat on a table with the smaller board in the middle and the longer edges close together. Sift corn meal， fine coal dust， powdered pumice， plaster of Paris and fine sawdust in even layers over the boards. Now lift carefully the inner edge of one of the wider boards and slip under it a narrow strip of wood 1 or 2 cm. thick. The layers of material spread over the boards will be broken and slant back from the line of breakage with their edges exposed along this line. Do the same with the wide board on the other side. The conditions shown will be similar to those exhibited in block mountains.
In southern Oregon and extending southward are found long， narrow mountain ridges， having a steep cliff on one side and a gentle slope on the other. Between these ridges are flat， troughlike depressions in which small lakes are sometimes found. The ridges are formed of thick layers of rock inclined at the same angle as the long slope of the ridge. The short slope of the ridge exposes the edges of these layers which have been broken across.
The débris slopes at the foot of the steep cliffs in some cases are slightly broken across in a direction parallel to the cliff. The steep cliffs sometimes face each other with a somewhat flat depression between， and sometimes the cliff on one ridge faces the long slope of the next. Some of the ridges are more gullied than others， showing longer exposure to erosion.
These ridges are due to strains which have broken the rock layers and elevated those on one side of the fracture above those on the other side， so that a steep fracture cliff has been formed with the rock layers slanting backward from its elevated edge. (Fig. 118.) Mountains of this kind are called block mountains. As is seen from the fracturing of the débris slopes， the movement of elevation is not yet completed.
Fig. 118.
As some of the ridges are more gullied than others， it appears that the fracturing did not take place all at one time， but that the more gullied ridges were formed first. Earthquakes are not uncommon in this region. These are caused by a small slipping along the fault line.
In Oregon these ridges are little eroded. They are simple in structure and young in age. The longer streams flow down the gentle slopes parallel to the surface of the rock layers and the shorter streams along the steeper slopes across the edges of the layers.
197.Folded Mountains.
Experiment 132. -To the long edge of a piece of board about 10 cm. wide and 20 cm. long tack securely one of the shorter edges of a piece of rather thick rubber dam about 20 × 25 cm. Tack the opposite edge to a strip of board about 2 cm. wide and 20 cm. long. Place the rubber dam thus arranged on a smooth table and secure the wide board firmly to the table by a clamp or nail. Taking hold of the strip， stretch the rubber dam as much as it will readily stand. Fasten the strip so as to hold the rubber dam in this stretched position.