Responding to the comments and criticism offered since the publication of his paper, "Some Relations in Origin Between Coal and Petroleum," embodying the so-called "carbon-ratio theory," published 20 years ago, the author discusses the progressive regional metamorphism of the mother rocks of coal and oil, which advances from stage to stage with each advance in orogenic movement, the periods of advance being, he holds, those of most active oil generation and of metamorphism represented by cracking, with probable underground enrichment or hydrogenation of oil in confinement. The extinction zone, or the oil "dead line," at which in their evolution the oils have become so light that they are no longer liquid at surface temperature and atmospheric pressure, is found in many re ions to fall in a zone narrower than at first defined and represents a carbonization (indicated by fixed carbon, pure coal basis) between 61 and 63 of the associated coals in those regions. Oil is believed to have been generated in several cycles, in all cases probably under dynamic influence. Relations of regional metamorphism to the "law of Hilt," which applies to oils as well as to coal, are discussed, together with sources of error, in defining the extinction zone.