Amongst hitherto described clay minerals, there are a number which have not yet been placed in clearly defined categories. In the course of an investigation of antigorites (Caillere, 1936), one of us has already shown that the thermal curves of bowlingites from Blackburn, Linlithgowshire, and Catkin Hills, Lanarkshire, of saponite from Catkin Hills and of diabantite from Bergen Hill, New Jersey, U.S,A., are very similar to the thermal curves of montmorillonites. We have therefore taken up again the study of these minerals with a view to determining more precisely their nature. We may recall that prior to the work already mentioned (Caillere, 1936), bowlingite and saponite were classed with antigorites, while the mineral from Bergen Hill, regarded as a diabantite, was classed with the chlorites.