Bentonites have been found in sites of different geological ages, but are more abundant in Cenozoic and Tertiary layers, the so called Cretaceous of Mesozoic period, or the Jurassic era, while they are rarely found in Paleozoic layers. They are generally assumed to have been formed through devitrification (weathering) of vitreous volcanic ash and tuff most often in the presence of water, followed by partial silica removal, and montmorillonite crystallisation. Their chemical composition depends on the type of water in which the volcanic ash had fallen.