The weilburgites of the Lahn region, western Germany, formerly termed diabase, are shown to have formed during the hydrothermal stage, and may therefore be termed "hydrothermalites." They are closely related to the keratophyres of the region, and it is suggested that they originated through the mobilization of the contents of the keratophyric magma basin by volatiles originating from an ascending diabase magma. The terms allopegmagenesis and allomagmagenesis are proposed for such a mode of origin, the latter designating the actual intrusion by an ascending magma into an intercrustal magmatic reservoir. The processes and their relation to rock composition and geologic age are discussed.