The solution alteration of a limestone depends on the rate at which water entering the rock becomes saturated. This rate is governed by a rate of transport of solute away from the solid-liquid interface, where the solution is always saturated. For laminar flow in the straight circular capillary of radius <i>a</i>, the solution will be 90 per cent saturated after penetrating a distance 0. 572<i>va</i>* h2*s/<i>D</i>, where <i>v</i> is the average flow velocity of the fluid, and <i>D</i> is the diffusion constant of the solute in the solution. Under normal conditions below the water table, <i>v</i> and <i>a</i> are sufficiently small that the water in a limestone is always saturated with respect to calcium carbonate. Solution alteration can therefore take place only where the water first enters the rock or where the solubility changes.