A black layer of accumulated dirt
If you leave your dead skin undisturbed, it will mix with sweat and dirt to 
form .... diet with vitamin D is particularly important if you have brown or black skin
,... [DOC] [TLE] Leeks  A Dirty Little Secret (Ingredient) - Today's Dietitian [PAR] Leeks also grow in layers, so as each new layer forms, it covers the underlying 
layer along with any accumulated dirt that may be there.