THE hypothesis of limestone assimilation1,2, developed to explain differentiation of the Vesuvius magma, was long unchallenged for that area—and extended3—even when it had to be abandoned for other alkaline provinces. Trace element data made it difficult to reconcile the limestone assimilation hypothesis with many observations4–7, but even in 1966 when Hurley et al.5 challenged Rittmann's hypothesis1 (on the basis of Sr isotope data) they had insufficient evidence to rule it out. But Pb isotope data support8 the genetic model of Hurley et al.5, and further evidence against Rittmann's hypothesis is presented here.