“…Boiling hydrochlorie acid solutions of various concentrations from N/lO (WOOD [1956]) to 10% (MCCALEB [1954]) have been recommended, but the effeet of these on apatite and other soluble minerals must be remembered. These coatings may eontain oxides of iron, aluminum or manganese, organie matter or amorphous silica, and they may act as a binding agent holding small particles of silt and clay size to the larger grains.…”