The colloidal properties and base-exchange characteristics of five deposits in four different areas were determined. The variations of the clay were, in general, a function of the amount of overburden and have been related to that dimension. From the standpoint of appearance, two types of clay were found in these areas. The highest grade clay has a yellow color whereas the other type, a blue clay, is customarily found to be of lower grade. Differential thermal studies of tile yellow and blue

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... y, when correlated with variations in colloidal properties, strongly suggest that the change from blue to yellow is due to oxidation of iron. The primary cause for variation iLL properties is due to the Na/Ca ratio in the exchange positions regardless of whether the iron in the clay is in the ferrous or ferric state. Examination of purified clays indicates that the exchangeable base ratio of the field bentonites lies in a zone of instability. This zone occurs when tile divalent ions amount to 40 to 60 percent of tile base-exchange capacity. 141