At many places in ‘The Peak District’ of Derbyshire, the igneous rocks, locally called ‘toadstones’, have, to varying extents, suffered an alteration with, as a result, the production of clays—locally called ‘toadstone-clay’. These clays attracted the attention of the present author some years ago, and since that time, whilst conducting extensive ‘field’ investigations in the course of a survey of the minerals of Derbyshire, observations on them have been ahnost continuously made and the deductions drawn therefrom carefully considered.