During crystallization of a large intrusive igneous body such as the Stillwater complex, crystals formed in the magma settle and accumulate on the floor. The resulting layer of hot solid tends to block heat flow downward from the magma. In this paper, the rate of heat loss from the magma is calculated, taking into account the effect of the settled crystals. For likely values of thermal conductivity and other parameters, the heat flux into the floor is too small to crystallize the magma trapped in pore space between settled crystals before they are deeply buried. This conflicts with mineralogical evidence that significant exchange of ions between the pore liquid and the main body of magma occurred during crystallization of the former, and with field evidence that the unconsolidated layer was thin. On the relevant time scale, diffusion could be effective over distances no greater than a few centimeters. Possible convective transport in the pore space is discussed, but this conflict is not satisfactorily resolved. Some remarks are included on convection and crystal settling in the magma.