“The rock forming the earth’s crust falls into three generic groups: igneous, sedimentary and metamorphic. Heat, pressure, and chemical reactions may change either igneous or sedimentary rock into metamorphic rock, meaning ‘changed in form’ usually into more compact and crystalline condition, and even metamorphic rocks may be further altered to higher ranks of metamorphism. Rocks may become plastic under great pressure and high temperature and by earth movement. They may be folded into complex forms with a banded structure. Many Constituent minerals may be dissolved, transported, and reprecipitated by thermal waters. Heat and pressure may cause recrystallization.

In this way, new rocks are formed, differing widely from the igneous or sedimentary types, and usually much harder than with. Thus shales and related rocks may be altered into slate. The shales from which slates originate were deposited previously as clay beds. There beds of shale, at first horizontal, were tilted by subsequent earth movements, and the intense metamorphism that converted these into slates folded and contracted them. Slate, then, belongs to the metamorphic group of rocks derived from clays and shales and possessing a cleavage that permits it to be split into thin sheets.”

- Marble Institute of America