Melting experiments on centimetre-scale blocks of tholeiitic Holyoke basalt from the Mesozoic Hartford basin of Connecticut reveal that when this basalt is only 30% crystallized, a relatively strong network of crystals exists. Cubes of the basalt melted to this degree retain their shape as the interstitial liquid drains easily from the crystal network. The intrinsic permeability of a sample containing 34% crystals was determined to be 3 × 10−10 m2, which is similar to that of coarse gravel. Serial sections through partly melted samples reveal that the network owes its strength to monomineralic strings of plagioclase and pyroxene crystals. Although these strings are clearly evident once the rock has been partly melted, they are difficult to see in thin sections of the initial basalt, but they are present. The compressive strength of the mush increases dramatically from 336 to 4333 Pa as the percentage of crystals increases from 33% to 37%. This rapid strengthening is believed to explain why sheets of segregation liquid in tholeiitic flood basalts have compositions that correspond to no more than 35% fractional crystallization of the host basalt. These sheets are proposed to form through compaction of the crystal mush in the lower part of thick flows, the compaction occurring only while the mush remains weak, that is, while it has <35% crystals.