Back-scattered electron microscope images of metasomatic calc-silicate rocks demonstrate complex mineral growth patterns; often oscillatory zoning defines a euhedral or subhedral morphology. We infer from this the existence of an appreciable transient porosity (up to a few percent) during metasomatic calc-silicate growth, and argue that porous, reacting calc-silicate layers become conduits for irreversible metamorphic fluid loss. Because the layers are too permeable to sustain a steep gradient in hydraulic head downdip, fluid pressure in the calc-silicate must generally be lower than in adjacent low-permeability lithologies, so that steep local gradients in the hydraulic head will drive fluid from surrounding rocks up, down, or sideways into the transiently porous reacting layer. So-called “metasomatic fronts” separate transiently permeable layers that have undergone metasomatism from little-altered, low-permeability rocks. Thus they develop parallel to the flow path at its edge and should be termed metasomatic sides.