The xenoliths display a range of isotopic compositions (¦Ä30Si=?0.43¡ë to ?0.15¡ë) comparable to that measured previously for igneous rocks. The isotopic compositions of the McBride xenoliths reflect assimilation and fractional crystallisation (AFC) and/or partial melting processes. Silicon and O isotopes are correlated in the McBride suite and can be explained by AFC of various evolved parent melts. In contrast, the Chudleigh xenoliths have Si isotope compositions predominantly controlled by the specific mineralogy of individual cumulates. Using the xenolith data and a number of weighting methods, the Si isotope compositions of the lower and middle crust are calculated to be ¦Ä30Si=?0.29¡À0.04¡ë (95 % s.e.) and ?0.23¡À0.04¡ë (95 % s.e.) respectively. These values are almost identical to the composition of the Bulk Silicate Earth, implying minimal isotope fractionation associated with continent formation and no light lower crustal reservoir.