GRAVITATIONAL METHODS 



363 



from the surface; (c) the maximum ridge between the two minima. The shift of the 

 center of the minimum eastward from the crest of the dome is presumed to be due to 

 asymmetry of the salt mass. 



Other excellent examples of gravity surveys on salt domes are given by Eby and 

 Clark.f 



Anticlines. — A map showing the gradient and curvature values 

 over an anticline in northern Mexico is given in Figure 150. This is a 

 very good example which also shows the effect of a fault cutting the 

 structure. 



Fig. 218. — Torsion Balance Survey across the Ft. Collins Anticline, 

 Larimer County, Colorado, October, 1928, under the direction of J. H. 

 Wilson. Contours on the Hygene member of the Pierre shale are taken 

 from U.S.G.S. Bulletin 796 B. 



A torsion balance survey of the Fort Collins Oil Field, in Larimer 

 County, Colorado, is shown in Figure 212. The gradient and curvature 

 values are plotted on a structural contour map made on the Hygene member 

 of the Pierre shale, as published in Bulletin 796 B of the United States 

 Geological Survey. 



Strong curvature values are indicated by the considerable length of the 

 R-line values, which lie in a direction at approximately right angles to the 

 anticlinal axis of the structure. This is believed to indicate that they are 

 under the influence of a strong regional effect related to the uplift of the 

 mountains not far to the west.| 



t J. B. Eby and R. P. Clark, A.A.P.G. Bull. 19 (3), pp. 356-377, March, 1935. 

 I J. H. Wilson, Colorado School of Mines Magazine, Oct., 1928. 



