The term 'Slate Source' refers to two different types of sources for slate, a type of metamorphic rock. The first source is shale, which is a sedimentary rock composed of clay and silt-sized particles. It can be found in many parts of the world and is often used as a source of slate. The second source is a QUARRY, which is an excavation site where rocks and minerals are extracted from the ground. Quarries are often used to obtain slate for construction and other purposes.