550 JOSEPH BARRELL 



from 60 to 70 km. apart would largely neutralize each other in 

 their surface effects. Furthermore, there is no notable extension 

 of the anomaly contours shown in any direction, and more espe- 

 cially at right angles to the line of stations, such as would suggest 

 the wider fields of force due to deep-seated masses. If the masses 

 were at great depth this hmitation of attraction to . ^^^ons near the 

 epicenters could be produced only by a special checkerboard 

 arrangement of opposite masses in all directions. It may be 

 rather firmly concluded, therefore that the anomalies of this chain 

 of stations along a line of low topographic relief are due to hetero- 

 geneities of density within the zone of compensation. 



In certain regions, as in Florida, in western New York and 

 Pennsylvania, and in the Great Basin, occur broad areas of anomaly 

 showing no central maximum. To some extent this is doubtless 

 due to incompleteness of observations, but in the areas mentioned 

 the stations are so spaced as to show that even if the map were 

 complete there would not exist marked domes of anomaly, such as 

 those central at Minneapolis, Minnesota, and at Lead, South 

 Dakota. This absence of domal form of anomaly curves suggests 

 that the disturbing masses cannot be below the zone of compensa- 

 tion, but should be interpreted as due to the effects of masses 

 widely distributed in the zone of compensation. This relation is 

 especially striking in southern Nevada. The deflection residuals in 

 northern Utah and Nevada all turn away from this southern area of 

 defective mass, shown in Fig. 5, Part II, of this article, as located 

 by Hayford and Bowie. Yet within this broad area of defective 

 mass Station No. 67 shows an anomaly of only —0.013 and some 

 of the surrounding anomalies have actually a larger negative value. 

 There is here then an entire absence of a broad domal form. This 

 is the region which indicates from the least-square equations of the 

 deflections of the vertical the shallowest compensation within the 

 United States; and the combination of the evidence from deflection 

 residuals and anomaly contours goes to show that the anomalies 

 are due to departures from isostasy within that shallow zone. 



An inspection of Fig. 5, Part II, shows furthermore that the 

 centers of plus and minus attraction as located by Hayford and 

 Bowie from the deflections of the vertical, although in general 



