Mud and clay vermiculations are irregular and discontinuous deposits of incoherent materials, almost ubiquitous. Found both inside and outside of caves, overlying limestone or other materials, they are formed from many substances (clay, mud, candle-black, colloidal silica, etc.). Also their shape dimensions vary greatly. The following genetical hypotheses have been proposed: fossil fillings; chemico-genetical deposition; biological formation; mechanical deposition from moving water or air; clay-layer drying process (Montoriol-Pous hypothesis); physicochemical deposition from drying liquid films. The last is proposed by the authors who, having discussed the various hypotheses, give many examples and the results of some experiments. They distinguish two types of vermiculations: Type 1 or negative vermiculations and Type II or normal vermiculations. The genesis of type I is explained by the Montoriol-Pous hypothesis; these vermiculations are large and made of clay or other colloidal material, and are due to the gradual drying of a layer of clay or other substance. The last stage of this drying process causes the vermiculations to form in a more or less dried state. The vermiculations of the second type are small and thin, much ramified and always with a clear "halo" around them. Vermiculations consisting of many materials have been observed, usually as macroscopic aggregates. They are caused by the drying of a liquid film containing suspended colloidal particles. The proposed mechanism provides a good explanation of all the observed characteristics of vermiculations.