198 



ROCKS AND SOILS 



formation of our solar system is that suggested by Dr. 

 Ross Gunn of the U. S. Naval Research Laboratory. 

 Dr. Gunn believes that our solar system was formed 

 by the splitting of a large star (see Fig. 318), one part 

 of which became our sun while the other part lost it- 

 self in space. According to this new idea certain forces 

 made the star rotate with constantly increasing speed 

 until it burst in two. When the two parts pulled apart, 

 according to the hypothesis, a long stem of matter was 

 formed between them. After a while this matter cooled 

 and gathered into the masses which formed the earth 

 and the other planets. 



What is the composition of the earth? The earth 

 consists of three layers of material : first, the atmos- 

 phere, a mixture of gases, which forms the outside 

 layer; second, the layer of water that covers approxi- 



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FIG. 319. IGNEOUS ROCKS AT GIANT'S CAUSEWAY, IRELAND 



mately three-fourths of the earth's surface ; third, the 

 solid earth. There is also an intermingling of these 

 three substances. The atmosphere contains tiny solid 

 particles in the form of dust, and particles of con- 

 densed water vapor. The upper crust of the earth 

 contains air and water. You learned many facts about 

 air and water earlier in your study of general science ; 

 therefore, we shall confine our attention in this study 

 mainly to the solid earth. 



The earth is about 8,000 miles in diameter, but man 

 has been able to penetrate the crust a distance of less 

 than two miles. The solid earth is composed of at least 

 two layers: first, there is a loose outer covering of 

 sand, gravel, clay, and soil called mantle rock; below 

 this layer of mantle rock is bed rock, which extends 

 to the center of the earth. 



The exact nature of the interior of the earth is not 

 known because we are unable to get to it. The earth 

 weighs about 5.5 times as much as a globe of water 

 having the same volume as the earth. This is just 



another way of saying that the density of the earth 

 is 5.5 times that of water. This is interesting because 

 the average weight of the upper crust of the earth 

 that has been explored is only 2.8 times that of water. 

 Because of this fact it is thought that farther down 

 in the earth there must be heavier materials. When 

 the nebular hypothesis was believed it was thought 

 that the core of the earth was a molten mass. Recent 

 scientific experiments indicate, however, that the cen- 

 ter of the earth is as rigid as steel. 



What are the main classes of rocks? If you have 

 carefully examined different rocks, you know that 

 they differ from each other in various ways. Some are 

 solid in structure ; others are made of separate grains. 

 The particles of which they are composed vary in size, 

 shape, hardness, and chemical composition. Some have 

 their particles or grains in layers ; in others the grains 

 are scattered ; some are entirely without a granular 

 structure. The nature of a rock depends largely upon 

 how it was formed. 



Rocks are generally grouped into three classes: 

 igneous, sedimentary, and metamorphic. Some of the rocks 

 of the earth were once molten. As they cooled they 

 solidified. This happens today when volcanoes pour 

 out their hot molten lava. When the molten matter 

 cools quickly it forms a glassy rock without any 

 crystalline structure. If cooling takes place slowly, 

 crystals are formed, and the slower the cooling the 

 larger the crystals become in size. Examine a piece 

 of granite. It is a good example of a crystalline igne- 

 ous rock. It is very hard and durable and is widely 

 used as a building stone and for monuments. All rocks 



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FIG. 320. STRATIFIED ROCKS AT WATKINS GLEN, NEW YORK 



