The light duſky blew concretes as well as the green, called by ſome Copper okers, yield a great deal of very good Copper when they are pure, which may be known from their colour and weight ; but thoſe that are more light are mixed with unmetallick earth, and thoſe that are yellow, containing iron oker ; on which account they are the more difficult to be met with, and yield leſs Copper of an inferior ſort.