378 H. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



from these data ; and, finally, to correlate the results so obtained with the 

 surface relief, and to compare these results with those derived from pen- 

 dulum observations, as to both the surface relief and the depth of the 

 so-called "zone of compensation." 



COMAGMATIC EEGIONS 



For some fifty years it has been recognized that the igneous rocks of 

 large areas show similarities in their chemical and mineral characters, 

 which indicate their derivation from a common magmatic source, and 

 which serve either to distinguish them from the rocks of other regions or 

 to bring out the similarity between them and others. This idea of the 

 existence of areas of related rocks is embodied in the terms "consan- 

 guinity' of Iddings, "petrographical provinces" of Judd, and the more 

 recent "comagmatic regions," a term suggested by me as expressing better 

 than the others the idea of derivation from a common magma. The ex- 

 istence of comagmatic regions and the apparently intimate relations be- 

 tween them and isostatic adjustment would seem to have been overlooked 

 by geodesists and geologists alike. 



The characters which may be regarded as peculiar to any given region, 

 and which serve to distinguish it from others, are, as yet, somewhat vague. 

 They are undoubtedly complex and are to be looked for in the ensemble 

 of the general characters of the rocks of a region, rather than as mani- 

 fested in any one rock. The most obvious comagmatic characters are 

 mineral — shown, for example, by the peculiar color of the pyroxenes or 

 amphiboles, the twinned structure of the feldspars, or the common occur- 

 rence in the region of minerals elsewhere very unusual. All such mineral 

 characters, however, are ultimately dependent on the chemical characters 

 of the magma or of the rock, so that the chemical characters are those 

 which are the most truly distinctive of a comagmatic region. These 

 chemical characters may manifest themselves in several ways. Thus, the 

 rocks of a region may be marked by the uniform presence of unusually 

 large amounts of one or more chemical constituents, as socla in the Chris- 

 tiania region, potash and lime in the Soman region of western Italy, 

 potash and soda in that of Montana, iron oxides in that around Lake 

 Superior, or lime and magnesia in the Urals. 



Again, comagmatic regions with essentially the same characters may 

 exist in widely separated parts of the earth, as in the Ancles, the Balkan 

 Peninsula, and the Eocky Mountains. The characteristics and the dif- 

 ferences are manifold, few regions have been submitted to systematic 

 study along these lines, and the technical details involved in a full dis- 

 cussion would carry us outside the scope of this paper. 



