358 ANNUAI. REPORT SMITHSONIAN INSTITUTION, 19 31 



To sum up, then, beginning at the surface and continuing down- 

 ward, we have, in order, a layer of granitic rock 12 km (7.5 miles) 

 in thickness; 25 km (15.5 miles) thickness of basalt; a 23-km (14 

 miles) layer of ultrabasic rocks; 1,140 km (700 miles) of silicon- 

 magnesium — the sima; a transition layer of silicon impregnated 

 with iron, of a total thickness of 1,700 km (1,060 miles) ; and, 

 finally, a great nickel-iron core of radius 3,470 Ian (2,150 miles). 

 The best marked discontinuities are those at 60 km and at 2,900 km. 



Figure 3 shows in schematic form the various layers. 



FiGUUE 3. — The .structure of the earth 



The discontinuities about which we should most like to know 

 more are those at 12 km, 37 km, and 60 km. We shall learn more 

 about them only through a study of earthquake waves and waves 

 generated by explosions. If you look over the edge and into a cup, 

 diagonally so as to just miss seeing a coin placed in the bottom of 

 the cup at the side nearest, and then pour water into the cup, the 

 coin becomes visible, due to the bending of the light ray as it passes 

 from the water into the air — a phenomenon of refraction. The echo 

 is a familiar example, in sound, of the phenomenon of reflection. 

 Earthquake waves are refracted in passing from material of given 

 density and elasticity into a second with different properties, i. e., 



