THE EARTH'S INTERIOR— ADAMS 265 



Underneath the granitic layer the crust, as indicated by the velocity 

 of earthquake waves passing through this region, is basaltic in com- 

 position. There is some evidence for a transition layer intermediate 

 in composition and position, but apparently the crust consists largely, 

 if not entirely, of the granitic and basaltic layers. 



By the mathematical theory of heat conduction the temperature 

 throughout the crust may be estimated if three principal factors are 

 known. These are (1) the age of the earth, (2) the average amount 

 of radioactive substances in the superficial rocks, and (3) the tempera- 

 ture gradient at the surface. The age of the earth — that is, the time 

 that has elapsed from the initial sohdification to the present time — is 

 probably not far from 2,000 million years, since minerals have been 

 found, whose age as determined by the lead-uranium ratio is 1,500 

 milUon years, and since from astronomical considerations an upper 

 limit of 3,000 million years is indicated. The average amount of 

 radium in the rocks found at the surface is about 3 parts in a million 

 milHon and the average temperature gradient in undisturbed regions 

 is 0.03° C per meter. Although these items are all subject to con- 

 siderable uncertainty, they allow us to construct a useful curve, show- 

 ing how temperature varies with depth. Especially striking is the 

 conclusion that below about 300 kilometers the temperature is nearly 

 the same as it was originally; the greater part of the earth is now as 

 hot as it was when solidification first took place. 



The accessible portion of the crust is almost entirely crystalline. 

 Glassy or amorphous material is comparatively rare. Wliether the 

 deeper parts of the crust are crystalline or glassy is a question upon 

 which there is not yet complete agreement. It is worth while to note 

 that the question cannot be answered by referring to the temperature- 

 depth curve already mentioned, because the only such curves that 

 have been constructed were based on the supposition that the termina- 

 tion of the era of free convection and the initial cooling of the earth as 

 a solid body were coincident with the freezing of the molten magma. 



The intermediate zone 2,000 miles in thickness extends from the 

 bottom of the crust to the top of the iron core (see fig. 2). Its striking 

 features are the major discontinuities in the velocities of earthquake 

 waves at the upper and lower boundaries. Passing from the crust 

 down through the upper surface of this region, longitudinal waves 

 suddenly increase their speed to 8 kilometers per second. The labora- 

 tory measurements on the elasticity of rocks, mentioned above, 

 indicated that only two kinds of rock could support so high a velocity 

 at the moderate pressures appropriate at a depth of about 50 kilo- 

 meters. These are dunite and eclogite, rocks that are found in few 

 places at the surface of the earth. Weighty evidence pointed to the 

 first of these as the more probable constituent of the intermediate zone, 

 and the conclusion was drawn that the shell consisted of this olivine 



