Careful analysis of the frequencies of heavy minerals in the Jurassic and Triassic sediments of York-shire, England, leads to the conclusion that mineralogical changes have been exceedingly active along a belt coinciding with the Cleveland axis. Many minerals are decomposed, others developed authigenically. Strata now appearing as an elongated dome were evidently deposited as successive lenses in a subsiding basin. Alteration of heavy mineral content is of such importance that serious mistakes in questions of provenance may be made if it is not properly taken into consideration.