266 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



rock and that therefore the whole earth, except for the iron core and 

 the relatively thin crust, was made up entirely of magnesium iron 

 orthosilicate. It followed that only four elements, silicon, oxygen, 

 magnesium and iron, composed the bulk of the earth's substance, all 

 the other elements being present in minor amounts. 



The Earth 



North R)LflR Cap 



A-60.KmT 

 B*2,900.KtS 

 C-3,'<0O.Km. ^„^,,-, 



FiouBE 2.— Section through the earth. (After Mohorovif 16.) A is the crust, B the intermediate eone, and 



C the central core. 



The force of this conclusion has been weakened somewhat by the 

 recent measurements of Birch and Dow, mentioned above, on a 

 specimen of diabase from Vinal Haven. The result obtained for the 

 compressibility leads to a wave velocity equaling that found in the 

 upper part of the intermediate layer, and thus appears to invahdate 

 the argument that the presence of dunite or peridotite in this region 

 is necessary in order to account for the observed velocities of earth- 

 quakes below the bottom of the crust. The method used was a linear 

 one, the cubic compressibihty being inferred from the change in length 

 of a small rod exposed to pressure, whereas at the Geophysical Labora- 



