COMAGMATIC REGIONS 



381 



crust) show very great differences in areal (not merely local) chemical 

 and petrographic characters. Some of the regions (as the Appalachian, 

 Ozarkiaii, and Coloradan) are essentially granitic or granoclioritic, others 

 (as the Algonkian and Oregonian) are gabbroic or basaltic, while still 

 others possess intermediate or divergent characters. In the second place, 

 consideration shows that the topographic and chemico-mineralogic fea- 

 tures of the several regions are, in general, interrelated. Areas of high 

 relief, as Appalachia and Colorado-Utah, are made up mostly of granitic 

 rocks, which are rocks of low density, while areas of low elevation, as the 

 Algonkian and the Oregonian, are composed largely of heavy gabbroic or 

 basaltic rocks. 



This relationship of high topography with rocks whose chemical and 

 mineral characters imply a low density, while the areas of low topography 

 are associated with rocks which on chemico-mineralogic grounds must be 

 heavy, is, as we have seen, exactly the relationship demanded by the 

 theory of isostasy. We shall find that such interrelations are generally 

 true, not only for the United States, but for all the regions of the earth 

 whose igneous rocks and surface features are sufficiently well known to 

 permit of correlation. It remains now to render our ideas more precise 

 and to place the correlation on as exact a quantitative basis as may be 

 possible, at the same time extending our study to all possible regions of 

 the earth. For this purpose we must arrive at a method of determining 

 the average density of the igneous rocks of the various areas or regions, 

 so that we may compare them with their average elevation above sealevel. 



The normative average Density 

 discussion of the methods 



Two methods are available for estimating the average density of the 

 igneous rocks of a region. The one is chemico-mineralogical, and con- 

 sists in obtaining first the average chemical composition of the igneous 

 rocks of the region, calculating from this the corresponding (average) 

 mineral composition according to a uniform method, so that the data for 

 different regions may be comparable with one another, and from these 

 mineral compositions calculating the average density, the densities of the 

 several constituent minerals being known. This method, which was first 

 used by Iddings, is here called the "normative method," because it makes 

 use of the so-called norm of the quantitative system of classification of 

 rocks as the mineral basis of comparison. The second method consists in 

 averaging all available densities as determined on the actual rocks of a 

 given region by the ordinary physical methods, as by weighing the speci- 



