GEOLOGY 



2437 



GEOLOGY 



Not one of the pupils had forgotten the story 

 of the origin of the earth as it is given in the 

 Bible: "In the beginning God created the 

 heaven and the earth. And the earth was 

 without form and void." Since these words 

 were written, their teacher told them, a num- 

 ber of theories have been advanced by astron- 

 omers and geologists to explain the beginnings 

 of the earth from a scientific point of view. 

 Practically all, however, start with the idea 

 that the earth and other planets of the solar 

 system were evolved from a nebula of some 

 kind. The theory known as the nebular hy- 

 pothesis (which see) for a long time had wide 

 acceptance. According to this theory the par- 

 ent nebula was a body of hot gaseous matter 

 that tended to take the form of a globular 

 rotating mass. As this mass gradually cooled 

 it contracted and threw off rings of rotating 

 matter, each of which formed a planetary 

 system. 



A more recent theory, the planetesimal hy- 

 pothesis, is now finding general favor with 

 scientists who have rejected the nebular hy- 

 pothesis because of certain grave defects. Ac- 

 cording to the newer theory the original nebula 

 consisted of swarms of small bodies called 

 planetesimals, which moved in orbits about 

 a common center. As these small bodies pur- 

 sued their slightly different orbits they came 

 together and were welded by heat and pres- 

 sure into those bodies we call the planets, 

 planetoids and satellites. This process went 

 on for ages, and the earth grew by capturing 

 other planetesimals. The great central mass 

 of nebula formed the nucleus of the sun. But 

 the pupils wished to know how they could 

 fill in the details. Were the mountains always 

 here? Where did the water that fills the ocean 

 basins originate? What is it like in the inte- 

 rior of the earth? This is what they learned. 



The Interior. Geologists infer that the in- 

 terior of the earth is hot because (1) hot 

 vapors and molten lava from beneath the 

 earth's crust are given out by volcanoes; (2) 

 hot springs, in some of which the water is at 

 the boiling point, are found in various parts 

 of the globe; (3) as men proceed downward 

 into mines and deep borings they find that 

 the temperature increases at the rate of about 

 1 F. for every fifty or sixty feet of descent. 

 It must be remembered in all discussions of 

 the earth's interior that man has as yet merely 

 scratched the surface. The deepest mines ex- 

 tend downward only a little over a mile, and 

 it is almost 4,000 miles- to the earth's center. 



Therefore the condition of the interior, that 

 is, whether it be molten or solid, is also a mat- 

 ter of inference. Some authorities have be- 

 lieved the nucleus of the globe to be in a 

 molten state; others think that except in cer- 

 tain local spaces, the earth is rigid and solid 

 from surface to center. The first theory, that 

 the interior is in a molten condition, is no 

 longer generally accepted because the earth, 

 in its relations to other members of the solar 

 system, acts like a solid rigid body. Also, the 

 average density 6f the earth is much greater 

 than that of the rocks at the surface, pointing 

 to an interior that is very dense and heavy. 

 It seems probable to most geologists that the 

 interior is solid, and so hot that under ordi- 

 nary conditions it would melt. Melting is pre- 

 vented, however, by the great load of the 

 earth's crust, the melting point of rocks being 

 raised by increase, in pressure. It is held by 

 many that the rock of the interior is molten 

 only in those places where the pressure is 

 relieved; that is, where the crust is upreared 

 under mountain folds. 



Air and Water. The origin of those two 

 spheres of the earth that we call the atmos- 

 phere and the hydrosphere cannot positively 

 be stated. Some of the theories once held 

 are regarded now as of no more value scien- 

 tifically than so many fairy tales. According 

 to the nebular hypothesis the primitive atmos- 

 phere was of vast extent, heavy and very hot, 

 and it contained ^IT the water of the globe, as 

 well as that portion of the carbon dioxide and 

 oxygen now found in the atmosphere and in 

 organic tissues. Also, these constituents were 

 abstracted by the cooling earth from the at- 

 mosphere, which thereby suffered gradual de- 

 pletion. It is not necessary here to go into 

 details as to why this theory is weak; it is 

 sufficient to say that a critical examination of 

 the biologic and physical aspects of the earth 

 shows that it rests on no foundation of fact. 



A more plausible theory has been worked 

 out by the geologists who hold to the plan- 

 etesimal hypothesis. In brief, it is as follows: 

 In the early stages of the earth's history it 

 held a very thin atmosphere which grew as 

 the earth grew, by capturing planetesimals 

 from the central nebulous mass. When vol- 

 canoes came into action they added to the 

 atmosphere by contributing to it, as they do 

 to-day, water, carbon dioxide and other gases. 

 As soon as the atmosphere became saturated 

 with water vapor the latter was condensed 

 into water, forming the beginning of the hydro- 



