Chap. 7] GRAVITATIONAL METHODS 75 



Weathering may produce appreciable changes in porosities and densities 

 of surface formations, which may have to be allowed for in torsion-balance 

 terrain corrections when they occur near the instrument. 



Densities of formations undergo considerable change when their condi- 

 tion is disturbed artificially. Consideration of this e£fect is important in 

 leveling torsion balance stations in hilly country. The volume weight of 

 soil or clay may be reduced as much as 50 per cent, as shown in 

 Table 9.' 



Table 9 



Bulk Denbttt or 

 Rock Rock in Situ Fill 



Trap 2.992 1.712 



Granite 2.720 1.552 



Sandstone 2.416 1.376 



In the course of their geologic history, sediments are submerged to 

 greater depth and subjected to gravitational pressure and diastrophic 

 forces which bring about an expulsion of excess water, a dehydration of 

 colloids, and a deformation and granulation of soft grains. This results 

 in an apparent increase of density with geologic age. The effect of gravi- 

 tational pressure on density and porosity may be determined for moderate 

 pressure from experiments on sands, clays, and muds. Hedberg' calcu- 

 lated the variation of shale porosity to be expected with variations in 

 overburden thickness. A number of other attempts have been made^ to 

 express changes in density with depth by a simple formula. However, 

 it is doubtful whether such relations, based on observations in one area, 

 are universally applicable for geophysical purposes, since the variable 

 effects of diastrophism cannot be separated from those resulting from 

 gravitational compaction. For instance, Hedberg's recent density-depth 

 curves^ (see Fig. 7-2) indicate a much smaller increase of density with 

 depth than do the curves published earlier for the mid-continent. Hedberg 

 concludes that it is best to use different expressions for the ranges to 800, 

 800 to 6000, and 6000 to 10,000 feet,^ but for practical convenience he 

 gives the following approximation formula for the entire range (except 

 the first 200 to 300 pounds): P (porosity) = 40.22 -0.9998^ where p (the 

 exponent) is pressure in pounds per square inch. 



s After A. C. Lane, Geol. Soc. Amer. Bull., 33, 353-370 (1922). 



' hoc. cit. 



•> W. W. Rubey, Amer. Assoc. Petrol. Geol. Bull., 11, 621-633, 1333-1336 (1927); 

 U. S. Geol. Survey. Prof. Paper, 166A, 1-54 (1930). 



L. F. Athy, Amer. Assoc. Petrol. Geol. Bull., 14, 1-24 (1930), ibid., Sidney Power 

 Mem. Vol., 811-823 (1934). 



» H. D. Hedberg, Am. J. Sci. 31(184), 241-287 (April, 1936), with very complete 

 bibliography on the subject. 



* Depth in feet and pressure in pounds per square inch are almost equal nu- 

 merically. 



