296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



2. The density of the surface rock is about 2.Y, while the density 

 of the earth as a whole is about 5.52. This means that the density of 

 the core is necessarily very high. Both the density and the character 

 of the earth's interior have been computed from the behavior of the 

 different earthquake waves in their passage through the earth and as 

 recorded on seismographs in many parts of the world. 



3. It is the general opinion of scientists that the outer crust of the 

 earth is a highly siliceous material, largely granitic in character, ex- 

 tending to a depth of 35 to 40 miles in the continental areas. Under 

 the shallow ocean basins it extends less deeply and is probably entirely 

 absent in the deeper portion of the Pacific. Below the granitic layer 

 and reaching to a depth of about 1,000 miles is basic igneous rock. 

 This basic division increases in density with depth and reaches at 

 base a density of about 4.5. The upper part of this division is basalt 

 and the lower part is peridotite. The basalt portion has in general, 

 at the present time, a crystalline upper layer, which is thicker under 

 the ocean, where heat escapes more rapidly than under the continents. 

 Beneath this crystalline layer the basalt is thought to be in a glassy, 

 highly heated, solid-fluid condition. 



From 1,000 miles to 1,800 miles there seems to be a gradual transi- 

 tion from peridotite to nickel-iron, with density increase from 4.5 

 to 9. 



From 1,800 miles to the center of the earth, at 3,980 miles, the 

 material is thought to be largely nickel-iron of a nonelastic character 

 and possibly fluid, since transverse earth waves do not pass through 

 it. At the center of the earth the density is estimated to be about 

 11.6. At the surface of the earth an equal mixture of iron and nickel 

 would have a density of about 8.3. 



4. The temperature in the crust of the earth, as recorded in deep 

 borings, shafts, and tunnels, shows an increase with depth at the rate 

 of about 1° Fahrenheit for each 60 to 70 feet. If this rate of increase 

 continues with depth, which is improbable, the center of the earth 

 would have a temperature of about 350,000° Fahrenheit. 



5. The outer portion of the earth is a poor conductor of heat ; thus 

 the surface temperatures are mostly controlled by the sun's heat. 

 The amount of heat radiated into space by the earth is greatly 

 increased by the close approach of the molten rock to the earth's 

 surface or during periods of vulcanism. 



6. Kadioactive materials in the earth occur in all types of rock but 

 more largely in the acid or granitic types. It is thought by many 

 that sufficient heat is derived from the slow decomposition of radio- 

 active minerals to melt periodically a considerable portion of the 

 basaltic layer, and locally the lower part of the siliceous crust. 



T. At the earth's center there must be a pressure of about 45,000,- 

 000 pounds, or 22,000 tons, per square inch. Under this heavy 



