The porosity and pore-size distribution of highly indurated carbonate rocks and igneous and metamorphic silicate rocks from the Valley and Ridge and Piedmont provinces in Virginia were studied by mercury-porosimetry, specific-gravity, and petrographic techniques. Porosity values were found to range from 0.19 to 3.66 percent, and mean pore diameters from 0.010 to 0.866 µ.

Fundamental differences in the character of pores were noted between the igneous and metamorphic rocks and the carbonate rocks. Pores in the individual crystalline rocks are concentrated predominantly at two to four different sizes, whereas those in the carbonate rocks generally are of one size. Also the mean pore diameters in the carbonate rocks are generally smaller; the pores are of a lesser size range than those in the igneous and metamorphic rocks, but the porosity of the carbonate rocks generally is higher and shows a greater variation. The low porosity and small pores measured in the dolomitic rocks from Virginia appear to indicate that most dolomitization preceded the strong compactive forces of Appalachian folding.