RELIEF FORMS OF THE LAND. _ 48 compared with the size of the earth. If repre- sented on an ordinary terrestrial globe, it would be scarcely discernible, since it would project above the surface only about the z5,th of the diameter. The highest elevations of the earth are proportionally much smaller than the wrinkles on the skin of an orange. 4000 miles, 2000 miles. 1000 « 500 « 250 « > Wy —— Fig. 39, Relative Height of Mountains, Tf, as in Fig. 39, a sphere be drawn to represent the size of the earth, its radius will be equal to about 4000 miles. If, now, the line A B be drawn equal to the radius, it will represent a height of 4000 miles. One-half this height would be 2000 miles; one-half of this 1000, and successive halves 500 and 250 miles. An elevation of 250 miles would not therefore be very marked. Although the irregularities of the surface are comparatively insignificant, they powerfully affect the distribution of heat and moisture, and conse- quently that of animal and vegetable life. An elevation of about 350 feet reduces the tempera- ture of the air 1° Fahr.—an effect equal to a difference of about 70 miles of latitude. High mountains, therefore, though under the tropics, may support on their higher slopes a life similar to that of the temperate and the polar regions. 103. The Relief Forms of the Land are divided into two classes : Low Lands and High Lands. The boundary-line between them is taken at 1000 feet, which is the mean or average elevation of the land. Low Lands are divided into plains ond hills. High Lands are divided into plateaus and mountains. If the surface is: Scoreparaticele flat or level, it is called a plain when its elevation above the sea is less than 1000 feet, and a plateau when its ele- vation is 1000 feet or over. 6 If the surface is diversified, the elevations are called hills when less than 1000 feet high; and mountains when 1000 feet. or over. . Plains and Hills cover about one-half of the land surface of the earth. In the Eastern Continent they lie mainly in the north; in the Western, they occupy the central portions. Plains generally owe their comparatively level surface to the absence of wrinkles or folds in the crust, in which case the general level is preserved, but the surface rises and falls in long undulations: these may therefore be called undulating plains. The flat surface may also be due to the gradual settling of sedimentary matter. In this case the plains are exceedingly level. They are called marine when deposited at the bottom of a sea or ocean, and alluvial when deposited by the fresh water of a river or lake. Alluvial plains occur along the lower course of the river or near its mouth. Marine and alluvial plains, from their mode of forma- tion, are generally less elevated than undulating plains. 105. Plateaus are generally found associated with the mountain-ranges of the continents. Their connection with the adjacent plains is either ab- rupt, as where the plateau of Anahuac joins the low plains on the Mexican Gulf; or gradual, as where the plains of the Mississippi Valley join the plateaus east of the Rocky Mountains. 106. Mountains.—In a mountain-chain the crest or summit of the range separates into a num- ber of detached portions called peaks; below the peaks the entire range is united in a solid mass. The breaks in the ridge, when extensive, form mountain-p asses. The influence of inaccessible mountains, like the Pyr- enees and Himalayas, in preventing the intermingling of nations living on their opposite sides, is well exemplified by history. In the past, mountains formed the boundaries of different races. Some mountains, like the Alps and the Appalachians, have numerous passes. A Mountain-System is a name given to ceveral connected chains or ranges. Mountain-systems are often thousands of miles in length and hun- dreds of miles in breadth. The Axis of a Mountain-system is a line extend- ing in the general trend of its chains. Where several mountain-axes intersect one an- other, a complicated form occurs, called a Moun- tain-Knot. The Pamir Knot, formed by the fo earueationl of the Karakorum, Belor, and Hindoo-Koosh Mountains, is an example. It lies on the southern border of the elevated plateau of Pamir.