GRAVITATIONAL METHODS 



291 



about 1 foot. The core is removed from the cylinder and put into a container, or on a 

 small sheet of canvas. The bottom of the hole is leveled, and small rocks removed. This 

 material is added to the core, and the total material removed from the hole is weighed. 

 The volume of the hole is measured by putting into it a thin cloth sack of slightly 

 larger diameter and of cylindrical shape. The sack is filled to the level of the original 

 ground surface v^^ith glass beads or small marbles (about %-inch diameter) or other 

 constant-volume, light-weight material. The volume of the material required to fill the 

 sack is then measured, in cubic centimeters. The density of the surface layer is calcu- 

 lated by dividing the weight (grams) by the volume (cubic centimeters). 



At the risk of generalization it may be said that most sedimentary for- 

 mations have been deposited in fairly large basins and their density remains 

 substantially constant, if undisturbed, for considerable distances laterally. 

 This makes the assumption of uniform density of beds and the continuity 

 of density contact surfaces, or surfaces of density contrast, valid to a useful 

 degree, giving a basis for interpreting gravity results. 



It is not implied that lateral changes in density may not give rise to 

 gravity anomalies, for such are of record. However, they are the exception 

 rather than the rule. In fact, if the density of the underlying materials is 

 not reasonably uniform (as may be the case in glacial drift, which changes 

 character very rapidly in short distances and in which boulders of various 

 sizes may be present) the measured values of gravity gradient and curva- 

 ture may be erratic and entirely useless. 



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Essential Features of the Torsion Balance 



Whenever the gravitational field is warped or distorted, the values of 

 gravity at neighboring points on the surface of the earth dififer both in 

 magnitude and direction. Thus, if two small 

 masses at different elevations are supported 

 at opposite ends of a beam which is sus- 

 pended by a torsion wire, the equipotential 

 surfaces passing through the two small 

 masses will not be parallel and the magni- 

 tudes of the force of gravity acting on the 

 two masses will not be the same. The non- 

 parallelism of the two equipotential sur- 

 faces through the two masses and the dif- 

 ference in the forces of gravity acting on 

 the masses create a rotational torque which 

 acts on the suspended system. The torsion 

 balance is an instrument for measuring this 

 rotational torque. In principle, the torsion 

 balance is simple. However, due to the 

 extreme precision of measurement re- 

 quired, it is a complex and highly refined 

 instrument. 



A schematic representation of an Eotvos 



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D- 



Fig. 152. — Schematic representation of 

 Eotvos torsion balance. 



