The rheidity of a substance is defined as that property which determines whether it will behave as a fluid or solid for a particular experiment. It may be measured for given conditions of temperature, pressure, and shear stress, by that time for which the shear must be maintained for the deformation by viscous flow to exceed by one thousand times the elastic deformation. When loads are maintained for longer than the rheidity, the substance deforms as a fluid, and the elastic terms of the deformation equation may be neglected as insignificant. The rheidity of ice, salt, gypsum and serpentine are respectively of the order of a fortnight, a year, ten years and ten thousand years. Glaciers, salt domes, gypsum extrusions, and post-magmatic re-intrusion of serpentine are examples of rheid behaviour. Geological and astronomical evidence indicates that the rheidity of the mantle of the earth varies from tens of thousands of years at the top to hundreds of years at the base. Since tectonic loads are maintained from ten thousand to ten million years, the mantle of the earth behaves as a fluid for all geotectonic phenomena. The rheidity of the crust varies from 105to 109years, and hence the crust behaves as a solid for many tectonic processes. Geosynclinal materials and orogenic zones have, in general, shorter rheidities. and many fluid phenomena occur. Crystalline schists undergo rheid folding in the cores of orogens. Rheid folding, in spite of its appearance of extreme complexity, obeys simple geometrical laws, the understanding of which allows the complexly attenuated and contorted folds to be projected and extrapolated from fragmentary data. The universal contortion of the Archaean gneisses, which is usually regarded as evidence of intense shortening, does not necessarily imply much shortening or intense diastrophism.