Slate

[Fr.] A kind of rock which splits into thin layers very readily. The largest slate quar¬ ries in the world are in Wales. Welsh slates are lightest and best. They are used for covering roofs, for cisterns, for mantelpieces, and for writing upon. Slate is very useful for roofing purposes, since it is cheap, light, and impervious to water. Slates are fastened to the rafters with nails, and are placed so as to overlap one another.