GEOPHYSICAL LABORATORY. 161 



be analogous distributions in the stars and nebulae, as indicated by the 

 spectroscope. 



The study of the average chemical composition of the igneous rock of 

 different areas brings out many interesting facts. While the several 

 continents resemble each other in general chemical composition, yet 

 there are notable differences among them; also, the averages of 

 different countries and parts of continents differ among themselves 

 still more widely. The averages of the United States and of Europe 

 (which are best known) most nearly resemble the general earth average. 

 The average composition of the rocks of the Atlantic and Pacific Ocean 

 floors is very different from those of the continents and of most of the 

 larger land areas. 



From the average chemical composition of the igneous rocks of any 

 given area it is possible to calculate the density of this portion of the 

 earth's crust. This has been done for the continents, for the floors of 

 the Atlantic and Pacific Oceans, and for many countries and larger 

 land areas. The average densities are found to vary within rather wide 

 limits. Thus, the average density of the earth's crust is 2.77, that of 

 Asia is 2.72, while that of the floor of the Pacific is about 3.00. 



There is an interesting and significant inverse relation between the 

 average density and the mean elevation above sea-level : the lower the 

 density (or the lighter the average rock) the higher the mean elevation. 

 This is generally true all over the earth and is most strikingly shown 

 along a zone around the earth in about latitude 45° north, which 

 crosses the United States, Europe, Asia, and the Atlantic and Pacific 

 Oceans. The curves of rock density and land elevation vary in 

 opposite sense with scarcely an exception. 



The establishment of this relation, which is based on a very large 

 number of data derived from many and widely scattered portions of the 

 earth, is of special interest in connection with the theory of isostasy; 

 namely, that the lighter portions of the crust tend to rise, while the 

 heavier tend to sink, so that the various portions of the crust maintain 

 a state of balanced equilibrium and are not sustained primarily by the 

 principle of the arch. Gravity observations with the pendulum or 

 plumb-bob suggested this concept of the crustal conditions, and this 

 most recent evidence is confirmatory of the theory, although the 

 subject is approached from such an apparently unrelated starting- 

 point as the chemical analysis of igneous rocks. 



Another feature of this year's program has been to bring together, 

 in convenient and accessible form, the considerable body of information 

 about silica and the silicates which has accumulated as the result of 

 experimental work during the last twenty years, but is still rather in- 

 conveniently scattered through the scientific periodicals. The need 

 for such a work has been felt by several groups, each interested in the 

 silicates from a different point of view. 



