310 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



better to bring out the parallelism. The sequence of the continents 

 and oceans is arbitrary. 



It will be evident from the Table IV and from the graphs that 

 there is a close relation between the average densities of the conti- 

 nental masses and of the ocean floors and their average elevations or 

 depressions. They stand in inverse relation to each other; that is, 

 the higher portions of the earth's crust are composed of the lighter 

 rocks and the lower portions of the heavier. When it is remembered 

 that these relations are shown by a very considerable number of 

 averages based on a very large number of trustworthy analyses (the 

 largest so far available) from all parts of the earth, the correspond- 

 ences are too striking to be explicable by an appeal to chance or 

 coincidence. This is even more obvious when we come to consider 

 the relations in greater detail, as we shall do presently. 



In discussing this subject it must be kept in mind that we are 

 dealing with the averages of large areas and of many analyses, so 

 that small and local details are lost. Thus a number of volcanoes 

 show flows of heavy basalt covering lower flows or inner cores of 

 lighter rhyolite or andesite. Again, it is not uncommon to find 

 sheets of heavy basalt capping plateaus or forming the summits of 

 their mountain remnants. But such apparent contradictions to the 

 general law shown above are but local and minor details, insignificant 

 as compared with the immeasurably greater masses of which they 

 form but topographic surface features. 



The general relations between rock density and elevation are also, 

 and possibly more strikingly, seen when they are presented in greater 

 detail, as is done in figures 4 and 5. 37 These are based on the average 

 densities calculated from the average chemical compositions of the 

 rocks of different countries and regions, as determined by Doctor 

 Clarke. These represent the general elevations and corresponding 

 average densities along two zones around the earth, the one roughly 

 between latitude 40° and 50° N. and the other between latitude 

 10° and 20° S. It is to be understood that the graphs are much 

 generalized, representing average densities and elevations, so that 

 there is little detail. 



The outer circle is that of sea level and the irregular line that 

 crosses it is a generalized graph of the land elevations and the ocean 

 depths. Though the positions in longitude are approximately cor- 

 rect, the vertical scale of these is not that of the earth as represented 

 by the sea-level circle, but the heights and depths are very greatly 

 exaggerated, otherwise the differences would not be perceptible in any 

 practicable illustration. The portions of the land surface are, how- 

 ever, all drawn to the same (exaggerated) vertical scale, while that 

 of the ocean depths is one-half of this. The elevations shown for 



87 For the suggestion of this method of presentation I am indebted to Dr. L. H. Adams, 

 ef the Geophysical Laboratory. 



