Kaolin, or china clay, unlike most of the other clays mined in the United Kingdom, is residual in character, not sedimentary, being formed in situ in the mass of granite by decomposition of the original felspar. Granite occurs over a very considerable area of Great Britain but only in the granite bathylith of S.W. England, in Devon and Cornwall, does sufficient kaolinisation occur to justfy extensive exploitation which has been proceeding now for over 180 years. The granite intrusion welled up from great depths under a cover of sedimentary rocks of much greater age, which cover had already undergone great tectonic stresses prior to the granite's arrival.