74 GRAVITATIONAL METHODS [Chap. 7 



varying percentages of iron, copper, and zinc-sulfide ore whose mean 

 density was 4.71. 



In some ores the densities are directly dependent on the iron, lead, or 

 copper content; the mean density may then be computed from chemical 

 analyses, as borne out by the correlation of analyses and densities in 

 Table 7.' 



3. Densities of sedimentary formations change with porosity, moisture, 

 and depth of burial. The porosity of a rock is given by the ratio of volume 

 weight and density. If bd is the dry volume weight ("bulk density"), and 

 b the density of the substance ("mineral density"), the porosity P = 

 1 — bd/b. The density of the sample (powdered, if necessary) may be 

 determined in a pyknometer and the volume weight by the method de- 



Table 8 



Igneous rocks (except pumistone) and metamorphics 1%- 3% 



Dynamo-metamorphosed sediments 1 -10 



Consolidated sediments 10 -30 



Unconsolidated sediments, mostly postcretaceous (except diatomaceous 



earth and peat) 25 -60 



scribed under paragraph A-1. Porosity of rocks depends on the degree 

 of consolidation in the course of their geologic history as well as on weather- 

 ing when exposed at the surface. Igneous rocks have smaller porosities 

 than sedimentary rocks. Table 8 gives a tabulation of average values for 

 unweathered rocks. 



In the last group, shales and clays show the greatest variation. Imme- 

 diately after deposition, muds may have porosities as high as 70 to 90 

 per cent, silts from 50 to 70 per cent, sands from 30 to 40 per cent. 

 Moisture affects their bulk density considerably. Hedberg has deter- 

 mined porosities and densities of clays and sands from many localities. 



2 After H. Reich, Handbuch der Experimentalphysik, Vol. XXV, pt. 3, p. 16 (1930). 



3 Ibid. 



*H..D. Hedberg, "The effect of gravitational compaction on the structure of 

 sedimentary rocks," A.A.P.G. Bull., 10(11), 1035-1072 (1926). 



