260 ANNUAL IlKPORT SMITHSONIAN INSTITUTION, 1937 



depth relation lies in the fact that solely from laboratory measure- 

 ments of the elastic constants of rocks (to which reference will be made 

 later) we may calculate the wave velocity in various types of rocks, 

 and thence, by comparison with the loiown velocities at several depths 

 below the surface, make important deductions concerning the nature 

 and composition of the material within the earth. It is as if the 

 earthquake waves, upon arriving at the surface, carry with them a 

 message telling not only how long they have been on the wa}'^ and 

 how deep they have penetrated, but also how fast they traveled at 

 each point of their path, and finally the nature of the material through 

 which they have journeyed. To be sure, the messages are in code, 

 but happily the code has been deciphered by the ingenious devices of 

 the mathematician. 



Turning now to those laboratory measurements that pertain to the 

 present subject, we note first the constant of gravitation, originally 

 determined by Cavendish in the eighteenth century. The most 

 recent published result, that of Hcyl,® is C.67X10"® in absolute units. 

 From the measured constant of gravitation we Icnow at once the total 

 mass of the earth, and thence by combination with its volume, the 

 average density of the globe. The value obtained, 5.52, is a very 

 important one to which reference will be made later. 



The chemical composition of the rocks in or near the earth's surface 

 has been subjected to exhaustive investigation. It is a striking fact 

 that, although about 1,000 different minerals are known, the im- 

 portant and essential igneous rock-forming minerals number only 

 about a dozen, and that although some 90 chemical elements have 

 been found in or on tbe earth, 11 elements make up 99K percent of the 

 earth's layers. These elements in order of their abundance are: 

 oxygen, silicon, aluminum, u'on, calcium, sodium, potassium, mag- 

 nesium, titanium, phosphorus, and hydrogen. The above conclusions 

 are based on the studies of Washington and others of nearly 10,000 

 chemical analyses of rocks, which show also that ths average igneous 

 rock found at the surface of the earth corresponds to a granite or 

 granodiorite. 



Of especial significance is the composition of meteorites. As a 

 result of many analyses of these strange visitors from outer space 

 we now know that they consist largely of impure metalUc iron and 

 basic silicates approaching olivine in composition. Absent, or present 

 only in minor amounts, are the characteristic constituents of granite, 

 such as lime, alumina and the alkalis. 



An indispensable item in the list of factors from which we hope to 

 reach definite conclusions concerning the earth's interior is the com- 

 pressibility of rocks. One of the earliest grants of the Carnegie 

 Institution of Washington was to F. D. Adams, at McGill University, 



• Heyl, P. R., Bur. Standards Journ. Res., vol. 5, p. 1243, 1930. 



