THE STRUCTURE OF THE EARTH 273 



Italy was made higher than the grade of the railroad required in order 

 that a too great temperature might be avoided, but even with this 

 precaution the heat in some portions was so intense as almost to 

 stop the work of excavation. 



The heat of the interior of the earth is known also from lavas 

 which reach the surface through volcanoes and fissures (p. 299). 

 Copper wire, which melts at about 2200 F.,has been fused when thrust 

 into molten lava. In one case, where a lava stream from Vesuvius 

 overflowed a village, brass was decomposed into its component 

 metals. It is estimated that the initial temperature of lava, when it 

 issues from Vesuvius, is probably more than 2000 . 



Theories of the Physical State of the Earth's Interior 



If the temperature of the earth increased uniformly from the 

 surface downward at a rate of one degree for every 60 feet of descent, 

 a temperature of 3000 degrees would be reached at a depth of about 

 34 miles. Such a temperature would be sufficient to melt all but the 

 most infusible rocks under the conditions existing at the surface of the 

 earth, but since the rocks at this depth are subjected to the enormous 

 pressure of the overlying rocks, the conditions are very different. 



(1). Internal Fluidity Theory. — Based on the assumption that the heat increases 

 regularly from the surface downward, it has been held that the earth is a molten globe 

 covered with a thin crust 25 to 30 miles thick. Although the belief in a molten in- 

 terior has a wide popular acceptance, there are a number of serious objections to it. 



{a) Effect of Increasing Density. — ■ The rate of increase in temperature is probably 

 not uniform, but diminishes with the depth. This is evident from the fact that the 

 average specific gravity of the rocks of the earth's surface is about 2.8, while that of 

 the earth as a whole is 5.5, so that the specific gravity of the central portion must be 

 at least equal to iron, which is 7.7, and is probably higher. This increase in density 

 is due to some extent at least to the great pressure of the overlying rocks, but may 

 also be due to the concentration of heavy metals within the center of the earth. It 

 has been suggested that metallic iron is to be found in greater quantity there than on 

 the surface. But whatever the cause, the effect of incr«asing density would be to 

 increase the conductivity of the rock, so that instead of a uniform increase of one 

 degree for each 50 or 60 feet of descent, at a greater depth 70 feet might be required 

 for a change of one degree, then 80 feet, then 100 feet, and so on. The increasing con- 

 ductivity of the rock alone would, consequently, carry the temperature necessary to 

 fuse rocks much deeper than 30 miles. 



{b) Effect of Pressure. — Another serious objection to the theory of a molten in- 

 terior is to be found in the effect of pressure on the melting point of rocks. Since nearly 

 all substances expand upon melting, they remain solid when subjected to pressure 

 which prevents expansion, and in order to melt them it is necessary to raise the tern- 



