In regions of intense mountain-folding carbonaceous shales have lost the oil which, presumably, they once contained in the form of "kerogen" or similar mother-substance, and which is still present in equivalent rocks outside the metamorphosed zone. What became of this oil? The newer information on mountain-building indicates that folding commonly took place at considerable depth underneath great overthrust sheets of rock. Under such conditions it is believed that the oil was distilled from the carbonaceous rocks in what was, virtually, a giant high-pressure cracking still, and that it found easiest escape laterally along the bedding, being driven out to cooler zones by the gases generated during the distillation.

In later geologic ages the overthrust sheets became worn away and the whole area broadly uplifted, bringing the rocks formerly subjected to distillation up to the surface and opening their porous beds to the inflow and artesian circulation of meteoric waters which caused extensive secondary migration of the oil.

In the zone of intense metamorphism no oil would remain, the carbonaceous rocks would be devolatilized, and carbon ratios would be high. In an intermediate zone where the rocks are partly devolatilized, some of the oil would have been distilled in place, and some would have migrated from adjacent zones of more intense metamorphism. In a broad belt outside the zone of active distillation such oil as is found would not have been formed in situ, but would have migrated from adjacent zones of distillation under the influence of gas pressure and artesian circulation.