Intersections may exist between a zone of chemical discontinuity (basalt-feldspathic peridotite or eclogite-garnet peridotite) and a phase transition zone (basalt-eclogite or feldspathic peridotite to garnet peridotite) at about the depth of the continental M-discontinuity. Variations in depth to the chemical discontinuity and in the positions of isogeotherms produce flexibility in orogenic models. Intersections between the two zones at depth could be reflected at the surface by major fault zones separating large structural blocks of different elevations. The effects of such intersections on the conventional hypothesis for the development of tectogenes, geosyclines, and mountains are considered. The intersections could provide a beginning and an end to the more vigorous changes in surface level occurring during an orogenic cycle.