XII 



The Crust of the Earth 221 



Earth, as a whole, is 5-5 ( 85). Unless the enormous 

 internal pressure of the weight of the Earth's mass were 

 counteracted the rock substance would be compressed into 

 less space, and the mean density of the Earth would be 

 greatly raised. The high temperature of the interior causes 

 the rock substance to expand against the pressure of gravity, 

 and so maintains the comparative low mean density which is 

 actually found. The great pressure in its turn counteracts 

 the effects of high temperature by raising the melting-point 

 of the rock substance ( 72), and so preventing it from 

 assuming the liquid state. Astronomical observations show 

 that the Earth behaves as if it were a solid ball, and Sir 

 William Thomson has calculated, from the imperceptible 

 tidal effect produced in the lithosphere, that it must be as 

 rigid as if it were composed throughout of solid flawless 

 steel. 



293. Volcanic Action. Volcanoes are conical moun- 

 tains in communication with openings in the Earth's 

 crust, which continually or occasionally throw out steam, 

 hot stones, or white-hot melted rock called lava. Professor 

 Prestwich believes that there are hollows in the lower part 

 of the Earth's crust full of molten rock, which is squeezed 

 out by the pressure exerted by the Earth's crust contract- 

 ing slightly as it cools. Mr. Mallett, on the other hand, 

 thinks that the heated interior of the Earth in cooling con- 

 tracts more rapidly than the crust, shrinking away from 

 it and leaving hollows, into which the solid rocks subside 

 with much straining and crushing. The motion of the 

 rocks converted into heat melts some of them, and the 

 cracked crust allows the hot fluid to escape. Other author- 

 ities point out that since the lithosphere is solid only on 

 account of the pressure of the crust upon it ( 292), any 

 relief of pressure produced by the shrinking in of the central 

 mass, or by the cracking of the strata above, must allow the 

 rock substance to liquefy suddenly and with explosive vio- 

 lence. All volcanic activity is accompanied by the emission 

 of great quantities of steam, to the expansion of which 

 geologists believe the great power of volcanic explo- 

 sions is due. It is probable that a good deal of under- 



