FROM THE SOUTHERN UPLANDS OF SCOTLAND AND

and Northern Ireland indicate that the I-S from the three localities shows differences in both K § content and the degree of Tschermak substitution of octahedral A13+ for Mg 2 § The K § content correlates with percentage expandability (%E) of I-S as determined by X-ray diffraction (XRD), but both these parameters how poor correlation with palaeotemperature as estimated by organic reflectance methods. Octahedral Mg 2 § content shows an inverse correlation with A13 § and both these parameters show a good correlation with estimated palaeotemperature. Although K-substitution has been retarded at Dob's Linn, the region subjected to the highest emperature, Tschermak substitution of A13+ for Mg 2 § has apparently continued. In these K-bentonites, Tschermak substitution appears to be a better indicator of palaeotemperature than K-substitution. The conversion of smectite to illite, via inter-stratified I-S, has been widely studied by XRD methods. Because of the difficulty in obtaining monomineralic separates, chemical investigations of the transformation have been primarily restricted to bentonites. Chemically, the reaction can b