Tuff
Many volcanoes stand near the sea, and the ashes cast out by them are mingled with the sediments that are gathering at the bottom of the waters. In this way ashy muds or sands or even in some cases ashy limestones are formed. As a matter of fact most of the tuffs found in the older rock formations contain mixtures of clay, sand, and sometimes fossil shells, which prove that they were beds spread out under water.