The Blue Hills Complex, which lies south of the Boston Basin in eastern Massachusetts, is composed of Cambrian sedimentary rocks and Late Ordovician alkalic plutonic and volcanic rocks. On the north, the Complex is separated by a fault from the Boston Bay Group, which very recently has been shown to be late Precambrian. On the south, the Complex is overlain unconformably by the Pennsyvanian rocks of the Norfolk Basin. The bedding and volcanic flow structures of the Cambrian, Ordovician, and Pennsylvanian rocks are essentially parallel and dip steeply south. I believe that the Blue Hills Complex is a Late Ordovician cauldron subsidence that was tilted during the Alleghenian Orogeny and was thrust northward over the Boston Bay Group.