All geologists are familiar with those changes effected in argillaceous rocks under the influence of heat from igneous intrusions, changes that result in the formation of such rocks as chiastolite-slate, andalusite-hornfels, etc. It is therefore somewhat surprising that the changes produced during the artificial heating of clays should remain comparatively unknown to the body of geologists, although such changes are produced every day on a large scale during the baking of clays for the manufacture of pottery of various kinds. These changes have, indeed, been very little studied from the mineralogical standpoint. It might at first sight be expected that any changes brought about artificially by the action of heat on a clay would compare somewhat closely with those resulting in an argillaceous rock when subjected to contact-metamorphism—at any rate in the normal case of the latter change—when there is no transference of material from the igneous to the sedimentary rock. Yet such is hardly the case; there are certainly points of resemblance, but there are also very notable differences. It must be made clear that the clays used in the experiments to be described were selected on account of their importance for certain industrial uses, and not on account of any special similarity to those argillaceous rocks usually affected by contact-metamorphism. They are therefore clays of a special type with certain chemical characteristics that distinguish them from the more commonly occurring, and therefore more commonly metamorphosed, argillaceous rocks. Nevertheless, the lithological correspondence is close enough to institute certain comparisons.