COMAGMATIC REGIONS 



379 



The idea of petrographic provinces or comagmatic regions originally 

 denoted a comparatively small area, but we are coming to consider as 

 comagmatic the rocks of areas which may cover hundreds of thousands 

 of square miles. As we shall see, even such areas of the first magnitude 

 as the continental masses or the ocean floors may, in a sense, be regarded 

 as different comagmatic regions. The factors of shape of the area and 

 the geologic time of intrusion or extrusion of the igneous rocks must also 

 be taken into account, but we must leave aside such details here. 



For the student of isostasy the chief interest of comagmatic regions lies, 

 not in their chemical or petrographical features, but in the evidence that 

 they afford of the crustal heterogeneity of the earth, both surncially and 

 presumably for a considerable distance below the surface, and in the cor- 

 roboration of the theory of isostasy which their study affords. 



It will be well to give an illustration of the distribution of some co- 

 magmatic regions, with succinct statements of their chief characters. 

 That chosen is the distribution of the larger and more important regions 

 of the United States, which is of special importance to us here because 

 the igneous rocks of this country are well known chemically, the gravity 

 constants and anomalies are well known throughout the extent of the area, 

 and we have fairly accurate estimates of the average elevations of the 

 several regions. On later pages the correlation between these three factors 

 will be pointed out, but for the present we shall note only the distribution 

 and general sizes and shapes of the main comagmatic regions, with their 

 chief chemical characters. 



In the extreme eastern portion, running generally parallel with the 

 Atlantic coastline, is a narrow belt which extends from New England and 

 the Maritime Provinces, with some interruptions, along the Appalachian 

 uplift into Georgia, and which may possibly be continued in the Ozark 

 uplift. This Appalachian region (and the Ozarkian) is made up chiefly 

 of granitic rocks, which are characteristically rather high in soda and 

 lime. The later extensive sheets of Triassic basaltic rocks, which are 

 rather high in iron oxides, may be connected genetically with the Appa- 

 lachian region. Scattered along the general Appalachian trend, and 

 mostly along its eastern slope, are numerous isolated and small occur- 

 rences of plutonic rocks which are characterized by their high content in 

 soda. These small areas occur in Ontario, Quebec, New England. New 

 Jersey, Virginia, North Carolina, Arkansas, and Texas, and there is little 

 doubt that they are the surface manifestations of a sodic comagmatic 

 region beneath, which may be connected with the Appalachian one. West 

 of the Appalachian region, in the eastern Mississippi basin and extending 

 into Pennsylvania and New York, are some small, isolated occurrences 



