410 II. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



after correcting for latitude, altitude, topography, and isostatic compen- 

 sation, are called positive when there is an apparent excess of mass or 

 density, and negative when there is a deficiency of mass or density. 



This is shown by plotting the co-anomalic lines, as has been done by 

 Bowie for the United States, and republished by Barrell. 34 When such 

 an isanomalic map is compared with the distribution of comagmatic re- 

 gions in the United States, some very striking correspondences appear. 



A long, narrow belt of negative anomaly runs parallel to the Atlantic 

 coast along the Appalachian region and is continued (with a break in 

 New York, eastern Pennsylvania, and northern New Jersey) into New 

 England. As we have seen, the rocks of the Appalachian region are rela- 

 tively light. To the east of this zone are patches of positive anomaly, 

 which may be supposed to be connected with the magma furnishing the 

 extensive Triassic traps of this area. West of it is another positive patch, 

 which corresponds with the sporadic occurrences of peridotites and other 

 heavy rocks in the region east of the Mississippi, as in Kentucky, Penn- 

 sylvania, and central New York. A small positive patch covers the Adi- 

 rondacks, which form an outlier of the rather heavy region of Ontario- 

 Quebec. 



Around Lake Superior there is an area of .very high positive anomaly, 

 which corresponds very closely with the very heavy, iron-rich rocks of the 

 Algonkian region. South of this a small elongated area of high negative 

 anomaly coincides well with the Ozark uplift, the igneous rocks of which 

 are of low density. Farther west the gravity relations become somewhat 

 more complex, as do the comagmatic relations, but there is a general 

 correspondence, shown in the broad area of negative anomaly which covers 

 the plateau region of Utah, Nevada, Arizona, New Mexico, and part of 

 Colorado, the rocks of which are of very low density. The small patch 

 of very high positive anomaly south of Puget Sound may well be con- 

 nected with the extensive basalt flows of the Oregonian region. 



Further detailed discussion of these correspondences must be left to 

 the future, but sufficient agreement has been shown to indicate that there 

 apparently exists in the United States a close correlation between comag- 

 matic regions (and hence their areal densities) and gravity anomalies. 



In closing this paper it is a pleasure to express my sincere appreciation 

 of the invaluable assistance of my colleague, Dr. Leason H. Adams. We 

 have discussed much of the matter together, and the paper owes a good 

 part of what value it may possess to his thoughtful suggestions. 



34 J. Barrel!: Jour. Geo!., vol. 2U. 1914. p. 153. 



