Why is a tennis clay court called clay court when is actually a shale court. When and who decided to use the term clay instead of shale?

There is more than one type of tennis court and clay is one of them. There is shale stone brick and clay. Clay is more popular in Europe and Latin America, but is not used in the U.S.A. The material used in the U.S.A. green clay or rubico is commonly called clay but is not the same as other courts in Europe. Although cheaper to construct than other types of tennis courts, the maintenance costs of clay are high because the surface must be rolled to preserve flatness.