In the present paper, which is an expansion of part of a former one,2 there is presented a study of the relations between the average densities of different areas of igneous rocks and their average altitudes, with the object of correlating these relations with the theory of isostasy. The outcome of the discussion is that the general relation holds good: the average density of the igneous rocks of a region varies in the opposite sense as the average altitude. The results of the study, therefore, harmonize with and corroborate the theory of isostasy.

Comagmatic regions, in which the igneous rocks are shown by their similar chemical and mineral characters to be derived from the same magma, are briefly discussed, especially as to their bearing on isostasy, and the distribution of comagmatic regions throughout the United States is described.

The normative method for arriving at the average density of a . . .

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