THE NORMATIVE AVERAGE DENSITY 391 



similar details, but the various differences show their respective influences 

 in the calculated densities. Thus Africa, with 2 per cent less silica than 

 lias North America, shows a lower density because of its greater content 

 in soda and its consequent richness in the specifically light albite mole- 

 cule, while Australia, with about the same silica as North America, is 

 decidedly denser because of the greater proportion of normative pyroxene. 



Normative Densities and Altitudes 



On comparison of the densities with the altitudes, it will be evident 

 that the two, in general, vary inversely, or that the specific volumes and 

 the altitudes vary directly. There are some irregularities, but the general 

 agreement is unquestionable. This may be seen in the figures for the 

 various areas, large and small, whose average chemical compositions and 

 densities have been calculated. 



Satisfactory estimation of the average altitudes of most of the various 

 areas has been a matter of great difficulty. Fairly consistent estimates 

 for the continents and ocean depths are given by Murray, Wagner, and 

 others, and Murray also gives estimates for some smaller areas. Gannett 

 gives data for the whole United States and for the several States. The 

 most reliable of all such data were used, reference to them being given in 

 connection with Table III. 



For many, if not most, countries there is a dearth of good, or even of 

 any, estimates of the average elevation, and the same is true of particular 

 smaller areas. An attempt was made to remedy this defect by estimating 

 the average altitude of some areas, for which no other estimate was avail- 

 able, from the hypsometric maps in the Times Survey Atlas of the World 

 (London, 1920). These estimates, as well as most of those of the smaller 

 areas, can claim to be nothing more than very rough approximations ; 

 they have been used only for lack of more accurate data. 16 



In Table III are given the average altitudes in meters and the calcu- 

 lated average densities for 47 areas, including the continents and the 

 floors of the Pacific and South Atlantic. The densities are shown calcu- 

 lated as of rocks both water-free (8 a ) and water-bearing (8 W ) ; but it may 

 be remembered that the former is that which I prefer and which is most 

 often referred to in later calculations. The number of analyses on which 

 the density calculations are based are also given. 



16 My thanks are extended to Mr. Alan G, Ogilvie for kind assistance in collecting data 

 as to altitudes. I Avould also thank Dr. A. G. Brooks for his kindness in having the 

 average altitude of Alaska computed at my request. 



