76 



GRAVITATIONAL METHODS 



[Chap. 7 



The pores of rocks are usually not filled with air alone, but also with 

 water, oil, and/or gas, of which, for geophysical purposes, water is the 

 most important. It is difficult to estimate the percentage of pores filled ; 

 50 per cent is probably a good average. Water content ranges from 20 to 

 60 per cent in moderate climates and is less in arid, greater in humid 



Depth of Overburden in Feet 

 Fig. 7-2. Relation of shale porosity and density to depth, Venezuela (after Hedberg). 



Table 10 



Rock Density j 



Igneous rocks 2 . 80 



Clays 2.69 



Sandstones 2.67 



Limestones 2 . 76 



Table 11 



Rock 



Dry Volume 

 Weight 



Granite 2.58 



Dolerite 2.89 



Basalt 2.87 



Serpentine 2.71 



Schist 2.65 



Shale 2.75 



Sandstone 2 . 23 



Permian and Triassic sandstone, av 2.07 



Eocene sandstone 1 .86 



Porous limes 1.91 



Wet Volume 

 Weight 6, 



2.80 

 2.51 

 2.35 

 2.64 



Wet Volume 

 Weight 



2.60 



2.90 



2.88 



2.71 



2.67 



2.72 



2.35 



2.27 



2.18 



2.20 



climates. The wet volume weight (natural density) 5„ may be computed 

 from the mineral density 5, the pore volume P, and the water content of 

 the pores tr: 



100 V ioo> 



5. = 5 - 



Since 5d = 5(1 - P), 



5ff = 5d + 



100 



ioo 



