20-Slate

Fine grained slightly metamorphosed rock. Slate consists of microscopically small quartz and mica grains that have been aligned during mild metamorphism to form a layered (foliated) texture called slaty cleavage. Most slate is derived from the sedimentary rock shale, but some is derived from basalt lava. Its use as a building material is due to its cohesiveness produced during metamorphism, and to its slaty cleavage, allowing it to be split along flat planes.