380 IT. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



of plutonic rocks that contain much olivine and biotite and which are 

 characterized chemically chiefly by low silica and high magnesia and iron, 

 with considerable potash. These appear to be the surficial extensions of a 

 general body of magma which underlies the greater part of this area. 



Farther north, occupying the Laurentian shield around Hudson Bay, 

 is a large and chemically little known region, whose igneous rocks are 

 apparently mostly granitic gneisses, with large intrusions of anorthosites 

 in the southern part; so that high silica and high lime may be said to be 

 characters of this Laurentian region. South of this, around Lake Su- 

 perior, is an area of (chiefly) basaltic rocks, with some granites. A 

 prominent character of this region, which may be called the Algonkian, 5 

 is the high content in iron shown by the basaltic sheets and flows. In 

 and near the Ozark uplift, in Missouri and Arkansas, are occurrences of 

 aranites, which have been little studied, but which seem to constitute a 

 separate Ozarkian region, the general chemical characters of which re- 

 semble those of the Appalachian region, with which it may be connected. 



As we go west the comagmatic relations become more complex, as do 

 the geological structures, but we can distinguish some fairly well marked 

 regions. The best denned of these, the Montanan, extends from within 

 southern Saskatchewan, through central Montana, into Wyoming and, as 

 small patches, probably into eastern Colorado. This region is character- 

 ized chiefly by its high alkalies, with about equal amounts of potash and 

 soda, and a high percentage of baria as a minor feature. In the extreme 

 northwest is the Oregonian region, with its vast basalt sheets in Oregon, 

 Washington, and Id alio, which are notable for their high content in iron 

 oxides (like the Algonkian region). Extensive rhyolite flows accompany 

 the basalts, but the relations of the two are not yet fully understood. A 

 large and complex comagmatic region occupies the great high plateau of 

 Colorado, Utah, and Nevada, with portions of Idaho, Wyoming, Arizona, 

 and New Mexico. It is probable that further study will enable us to sub- 

 divide this great area into several distinct regions or subregions ; but, 

 taken as a whole, the chief chemical feature of this area is its medium or 

 average character, with rather high silica, soda, and lime, little potash 

 and moderate iron and magnesia. Finally, along the Pacific coast there 

 appears to be a narrow zone of distinctly sodic and rather highly silicic 

 rocks. 



From this brief survey of the chief comagmatic regions of the United 

 States, two facts which have a bearing on isostasy are evident. In the 

 first place, the igneous rocks (which constitute almost all of the upper 



r am indebted to Dr. A. C. Lane for the suggestion of this name. 



