EARTH S CRUST WASHINGTON. 



317 



granites, of low specific gravity, shows negative anomaly, and this 

 area is continued down along the Appalachian region, the rocks of 

 which are of general low density. The small areas of posi- 

 tive anomaly in eastern Massachusetts, Connecticut, and New Jersey 

 may probably be connected with the extensive Triassic flows of heavy 

 diabase and basalt which are so common here. Around the Lake 

 Superior region we find an area of very high positive anomaly, which 

 corresponds with the high density of the rocks of this region. 

 On the other hand, in the Missouri-Arkansas-Oklahoma region is an 

 area of pronounced negative anomaly, which corresponds with the 

 low density of the region of the Ozark uplift. East of this, 

 in Kentucky and Tennessee, is an area of somewhat high positive 





105° 100° 95" 



- b5 °- +C0$?80. 



Fig. 6.— Dotted portions : Areas of positive anomaly, excess of mass. Due to surface too 

 high or density of upper crust above mean. Blank portions : Areas of negative anomaly, 

 deficiency of mass. Due to surface too low or density of upper crust below mean. Grav- 

 ity anomalies in the United States. 



anomaly, and this is in harmony with the supposition made above 

 that the rocks underlying this area are decidedly heavy. 



Toward the west the relations become more complex, just as is the 

 geological structure and as are the comagmatic relations. We can 

 not here go into details, which are reserved for a future publication, 

 but the general area of negative anomaly covering the Colorado and 

 Nevada-Utah plateau, consonant with the light rocks here, may be 

 pointed out, as also the small area of high positive anomaly in Wash- 

 ington and Oregon, which may be connected with the very extensive 

 flows of heavy basalt of the Snake and Columbia Rivers. 



More might be said on this topic, but sufficient has been brought 

 out here, it would seem, to show that there is a coincidence, even to 

 very localized relations, between the average densities of comagmatic 



