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The Eastern Layered Intrusion of the Rum Layered Suite\ud comprises paired peridotite and allivalite (troctolite and gabbro)layers forming 16 macro-rhythmic units. Whereas the majority of these macro-units are believed to have formed by a process of crystal^liquid differentiation involving successive accumulation of crystals from multiple picritic replenishments of the chamber, the Unit 9 peridotite is interpreted as a layer-parallel picrite intrusion. Closely correlated with this discontinuous peridotite body\ud is a distinctive feature generally known as theWavy Horizon, which divides the overlying allivalite into a lower troctolite and an upper gabbro along a well-defined undulating contact. We propose that the Wavy Horizon is a metasomatic feature formed consequent to the removal of clinopyroxene from an original gabbroic mush.\ud Foundering of the mush into the picritic sill resulted in the replacement of the original interstitial liquid by one saturated only in olivine ( plagioclase). Progressive through-flow of this liquid resulted in the stripping out of clinopyroxene from the lower parts of the allivalite.We interpret theWavy Horizon as a reaction front, representing the point at which the invading liquid became saturated in clinopyroxene.The distinctive pyroxene-enriched zone immediately above theWavy Horizon could have formed when\ud mixing of the interstitial liquids on either side of the reaction front formed a supercooled liquid oversaturated in pyroxene, as a result of the curvature of the olivine plagioclase clinopyroxene cotectic. The presence of many such approximately layer-parallel features, defined by differences in pyroxene content, in the Eastern Layered Intrusion of Rum suggests that such an infiltration^\ud reaction process was not unique to Unit 9.