CHAPTER 4
THE EARTH"S CRUST
36.Land and Water Areas. -The surface of the earth has an area of about 197,000,000 square miles, about 28 percent of which is land. Such areas are too vast for us to conceive, but it may help us toward a conception to know that the area of the United States, exclusive of Alaska and islands belonging to it, is about 1/18 of the land area of the earth.
LAND AND WATEE HEMISPHERES.
Notice that London is about at the center of the land hemisphere.
It is possible to divide the surface of the globe into two hemispheres, one of which contains the larger part of its land and the other the larger part of its water surface. This bunching of the land has brought thepeople of the earth near together and has greatly facilitated their intercourse, especially since land transportation has become so easy. Under the climatic conditions which exist, it is very advantageous also for the inhabitants of the earth that the greater part of the polar lands are around the north pole instead of the south. The fact that the land masses have irregular outlines and are separated by water areas instead of being in one continuous extent is also, as we shall see, of benefit to the earth"s inhabitants.
37.Interchange of Land and Water Areas. -It has been foundfrom numerous observations that the land and sea do not always main- tain the same relation to each other. Areas which at one time wereland have since become sea and those which were once sea are now land. Sea shells are found imbedded in the rocks far from the sea and old river valleys are found by soundings under the sea at considerable distances from the present mouths of the rivers. What were once sea beaches are now found hundreds of feet above the sea.
From some such marks on the coast of northern Sweden it appears that the coast has risen about seven feet during the last 150 years. The Netherlands are sinking. Observations along the coast of Massachusetts give reason to believe that it is sinking very slowly. Indications of the movement of the land in respect to the sea are found in all parts of the world.
OLD SEA BEACHES, SAN PEDRO.
Three old beaches can be distinctly seen on the promontory.
Old sea beaches are found rising one above the other along the entire slope of a high hill at San Pedro, near the port of Los Angeles, California. Suess, the great Austrian geologist, thinks that the great changes of level between the sea and the land are due to a rising and falling of the sea and not a rising and sinking of the land. However this may be, there have been marked changes of level between the two and the boundary between sea and land has been a varying line. Sea and land areas have frequently interchanged, although deep sea bottoms were probably never dry land.
38.Characteristics of Land Surfaces. -The surface of the landdiffers from that of the sea in being at least comparatively immovable.
OLD ROOK BEACH, IMPERIAL VALLEY, CALIFORNIA.
Formerly part of the coast line of the Gulf of California.
It is rough and irregular, and is composed of many differ- ent kinds of rocks and soils. For the larger part of its area it rises above the level of the sea, but in a few places it sinks below, as in the Salton Sea, a part of Imperial Valley, California, and near the Dead Sea. Its surface is eroded by wind and water and is thusconstantly but slowly changing its features. Travel upon the land, overmost of the earth"s surface, is difficult because of the irregularities.
SALT WORKS ON THE SHORE OF THE SALTON SEA.
In 1905 the Colorado River broke through into this depression, which is below sea level, and completely covered the salt works seen in the picture.
Surface conditions also vary greatly over small areas. Great temperature changes occur on the land between day and night and between summer and winter. Land animals must exert considerable muscular force to move about, yet they must all move to get their food. They must therefore be highly organized to maintain themselves upon the land. Water animals are not subjected to the same difficult conditions. In fact, the conditions of life on sea and land surfaces are entirely different.
39.Characteristics of Water.
Experiment 35. -Place in a dish of fresh water a density hydrometer, or stick loaded with lead at one end, so that it will float upright. Mark with a rubber band the point to which the hydrometer sinks in thewater. In a dish sufficiently deep for the hydrometer to floatdissolve a considerable quantity of salt in water. After the salt has become thoroughly dissolved taste the water at the top and then after pouring off the larger part of the water taste that at the bottom of the dish. The salt is present in all the water but the appearance of the water has not changed.
Now place the hydrometer in the water containing the dissolved salt. It does not sink to the same depth that it did inFig. 41.
the fresh water. What can be said about the buoyancy of waterwhich contains substances in solution? Does a fish need to exert muscular force to float in water? Since water contains many substances in solution, it is possible for a water animal that does not move to be continually supplied with food.
Experiment 36. -(Teacher"s experiment). Place a small handful of zinc scraps in a strong wide-mouthed bottle. Fit the bottle with a two-hole rubber stopper having a thistle tube extending through one hole and a bent delivery tube through the other. The thistle tube should reach nearly to the bottom of the bottle. Connect the delivery tube with the shelf of a pneumatic trough by a rubber tube. Have several inverted 8 oz. wide-mouthed bottles filled with water on the shelf of the trough. Pour enough water through the thistle tube to partly cover the zinc and then pour on commercial hydrochloric acid or sulphuric acid diluted 1 to 10.
Chemical action will take place between the zinc and the acid and hydrogen will be freed. Allow the gas to escape for several minutes, as this is largely the air which was in the bottle. Collect several bottles full of the hydrogen. Keep the bottles inverted.