The mutual concordance of the various elements of the Sudbury lopolith, Ontario, and the overlying sedimentary rocks, and the gravity data, indicate the complex to be essentially a folded double sheet of granophyre above norite. The lopolith lies with complete discordance upon plutonic and metamorphic rocks of complicated structural and metamorphic history, and is overlain concordantly by rhyodacite tuffs which are intruded by the granophyre and are in turn overlain concordantly by sedimentary rocks. The granophyre ("micropegmatite") and the overlying tuffs are strongly alkaline, and share the distinctive chemical composition - low Al 2 O 3 and CaO, high Fe - that characterizes lopolith-cap rocks elsewhere. The tuffs may be the true cap of the complex, which may have been essentially extrusive in origin. The norite of the lopolith is unlayered and contains abundant interstitial micropegmatite, and is like the upper parts of other great lopoliths. The Sudbury basin may contain only an overflow part of the N. flank of a huge lopolith, the bulk of which has been removed by erosion and which contained the missing mafic and ultramafic differentiates.