Summary: An illite separated from an Upper Old Red Sandstone sediment at Denholm Hill Quarry, Roxburghshire, when examined by optical, X-ray, and chemical methods, appeared to be closely similar to Fithian illite. Its composition was 1·96M+(Al2·86Fe0·313+Fe0·092+Mg0·68)(Si6·92Al1·08)O20(OH)4, where M+ for the Ca-saturated sample was [(H3O)0·58Ca0·10Na0·06K1·12]. The differential thermal curve showed two peaks: one at 550° C. and the other at 600° C. (i.e. somewhat intermediate between the Fithian and Ballater illites). The dehydration curve was also intermediate. Electron-microscope examination showed rather better crystallization than in Fithian illite. The possible reasons for variation of the illite peak temperature are discussed.