268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



CONCLUDING REMARKS 



The problem of the earth's interior has not yet been solved. 

 Although much is known about conditions far within the earth the 

 interior still holds many mysteries. An explanation of the mechanism 

 by which deep-focus earthquakes occur is lacking, and we have no 

 clues as to the underlying cause of the numerous minor discontinui- 

 ties in the earthquake velocity-depth curves. Better knowledge of 

 the temperatures within the earth would be of great utility and, 

 especially, some basis for a more precise estimate of the original tem- 

 perature of the molten earth. A general understanding of the great 

 intermediate zone is essential before we can make satisfactory progress 

 in investigations of the crust. 



But although much remains for the future, we are able to point to 

 a number of definite accomplishments during the past several years. 

 These are (1) precise measurement of the elastic constants of rocks 

 and the determination of the speeds with which elastic waves will 

 travel through them; (2) the identification of the upper half of the 

 crust as a granitic layer; (3) a demonstration that the core of the earth 

 contains a heavy material such as iron; (4) an explanation of the two 

 major discontinuities within the earth in terms of the elastic constants 

 of typical rocks ; (5) the supplying of strong evidence that large masses 

 of iron may exist in the interior without influencing the earth's mag- 

 netic field, and (6) the establishment of an improved temperature- 

 depth curve for the crust and the region immediately below it. 



In attempting to paint a picture of the deeper parts of the earth, 

 the best we can do at present is to draw the outlines. Perhaps future 

 developments may enable us to make a bolder drawing and even to 

 fill in something of form and color. 



