232 JOSEPH BARRELL 



Now assume the unit masses at I and III to be parts of masses 

 of like density extending to the left of o to a distance N. Consider 

 the aggregate effect upon a given point, as that at 0.50, or in 

 general at point R. The effect of each unit at distance x to the 

 left of o upon the point at o. 50 will be measured by an ordinate at 

 a distance x to the right of o . 50. This will give the same aggregate 

 result as concentrating the masses at o and summing up the area 

 of the curve to the right of the point at o . 50 to a distance of o . 50+ 

 N. Stated in general terms, masses at depths I and III extending 

 linearly to distance N to the left of o will have an aggregate effect 

 upon a point R equal to the area of the curve between R and R-\-N. 



As to the aggregate effect on Fv, the gravity anomaly: If the 

 two sheets are of negative density, it is seen that the result will 

 be an increased negative anomaly over the effect of the separate 

 unit masses. If the lower mass is, however, of positive density, 

 the result for ordinarily Hmited sheets will be a change between 

 o and 0.50 from a large negative to a small positive anomaly. 

 This may be compared with the effects of other possible distribu- 

 tions of mass upon the gravity anomaly. 



If the anomaly due to the adjacent departure from uniform 

 distribution is of the mean value or greater, the more distant abnor- 

 mal masses will have but relatively small influence. This is because 

 the higher anomalies, with the exception of Seattle, are but two 

 or three times the mean. Further, in a zone of large radius there 

 are a greater number of positive and negative departures. Their 

 aggregate effect, according to the laws of chance distribution would 

 increase but slowly and this effect is diminished by distance accord- 



Fv=X.di,n 6 cos^ 6 

 mg to the formula ^^ . 



A reversal from a large anomaly of one sign to a large anomaly 

 of opposite sign, rather than a small one of opposite sign, marks 

 then in general a passage from an area of excess or deficiency of 

 mass to the opposite. A gradual change in the anomaly is the 

 reflection of a change in the subsurface abnormalities nearly as 

 gradual. If the areal variations show that the passages of the 

 anomaly through zero are not frequent, they go to show that 

 limited notable irregularities of density of opposite sign in the 



