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[ht]2sandstoneszw_big,limestones500kzw_big,drywetall_big,wg10040010003000zw_big width=2.75in,height=2.25inExamples of the correlation of slopes with porosity: (a) three sandstones (Murphy, 1982; 1984; Knight and Nolen-Hoeksema, 1990), (b) five limestones (Cadoret, 1993; Cadoret et al., 1995; Cadoret et al., 1998), (c) 11 fused glass-bead samples (Berge et al., 1995), (d) Westerly granite (Nur and Simmons, 1969) at four pressures. The observed trend is that high porosity samples generally have lower slopes than lower porosities on these plots, although there are a few exceptions as discussed in the text. These trends are easily understood since the slopes are determined approximately by the average value of v_s² for each material, which is a decreasing function of porosity.