HOW ROCKS WERE FORMED 



197 



the earth. They had no telescopes, spectroscopes, or 

 cameras. Airplanes, automobiles, and giant ocean lin- 

 ers had not yet been developed for making long jour- 

 neys. Today scientists are constantly making observa- 

 tions of the heavens and are studying the rocks, the 

 atmosphere, and the oceans with scientific tools and 

 instruments. As a result much scientific knowledge 

 has been gathered about the history of the earth, its 

 age, and its development, but much work remains to 

 be clone. 



The exact age of the earth will probably never be 

 known, but from facts gathered by patient scientific 

 work it is certain that our earth is very old. The latest 

 estimates place its age between 2,000 million years 

 and 3,000 million years. It is known that the earth is 

 constantly changing, there being no two seconds when 

 it is exactly the same. These changes seem to take 

 place in an orderly manner, each effect being preceded 

 by a natural cause, thus indicating that law and order 

 prevail. We shall learn a little later about some of the 

 forces constantly at work producing these changes. 



Courtesy Science Service 



FIG. 318. GUNN'S HYPOTHESIS 



How was our earth formed? When the writer was 

 a boy he was taught that the earth and all the other 

 planets were formed from a great hot, rotating mass 

 of gaseous matter that originally occupied the entire 

 space of our solar system. Each planet was thought 

 to have been formed from a gaseous ring that sepa- 

 rated from the cooling and shrinking parent mass. 

 The gaseous ring which formed the earth first cooled 

 into a liquid and eventually cooled enough to form a 

 crust of solid rock over a melted interior. This hy- 

 pothesis, known as the nebular hypothesis, was ad- 



vanced by Laplace, a great French mathematician and 

 scientist, in 1796 and was believed generally by scien- 

 tists for more than a century. See Figure 317. 



According to the nebular hypothesis : 

 The earth was once larger than at present. 

 The earth was once very hot. 



The earth became smaller by cooling and contracting. 

 The atmosphere of the earth was once much heavier 



than now. 

 The earth is still cooling. 



About the beginning of the present century scien- 

 tists began to doubt the truth of the nebular hy- 

 pothesis because many things about our solar system 

 could not be explained by it. Then the planetesimal 

 hypothesis was proposed as a better explanation of 

 the formation of our solar system. According to this 

 hypothesis the earth was formed hundreds of millions 

 of years ago as a lump of matter in a spiral nebula. 

 Look at the photograph of a spiral nebula (Fig. 289). 

 Many similar nebulae can be seen in the heavens now 

 with a telescope. Notice that much material is con- 

 centrated in the center, and extending outward are 

 curving arms. Dense knots of matter are spaced at 

 intervals along the curving arms. Scattered through- 

 out the arms are smaller knots of matter and count- 

 less numbers of tiny particles called planetesimal s. All 

 the knots and planetesimals move in orbits around 

 the central mass. Collisions occur, causing the larger 

 knots to grow at the expense of the smaller knots and 

 tiny particles. 



According to the planetesimal hypothesis the earth 

 at first was much smaller than now. With the passage 

 of time planetesimal matter fell into and collided with 

 the earth, which slowly increased in size. At first it 

 was probably not large enough to hold an atmosphere. 

 But as it slowly grew in size, its gravitational force 

 increased until it was able to hold an atmosphere con- 

 taining water vapor. As water vapor condensed some 

 of it flowed into depressions in the earth, beginning the 

 formation of oceans. 



The growth of the earth is now nearly complete. 

 Some of the brilliant meteors, commonly called 

 "shooting stars," that streak across the sky on a clear 

 night, add slightly to its mass. 



According to the planetesimal hypothesis: 

 The earth was once smaller than at present. 

 The earth was once colder than at present. 

 The earth became larger by the addition of planetesi- 

 mal matter to it. 



The temperature of the earth increased as it grew 

 larger because of the heat formed by increased com- 

 pression. 



The atmosphere of the earth was once much lighter 

 than now. 

 The most recent explanation offered to explain the 



