A description is given of the widespread development in the U.S.S.R. of deposits of clay, largely kaolinitic in mineralogical content. These deposits also may be traced outside the U.S.S.R. Their origin is believed related to a period of intense weathering in Mesozoic times. It is pointed out that this thesis of an ancient crust of weathering would well repay application in other countries, where deposits normally regarded as of pneumatolytic origin might possibly be due to intense weathering.