In the present paper a systematic classification of ancient calcareous materials has been proposed which more closely defines limestone in terms of the ratio of calcium to magnesium present. The same classification may be applied to mortars and plasters to characterize the raw materials from which they were made. The classification includes "pure" calcium and magnesium minerals and any combination of the two.

A new analytical procedure has been devised which eliminates the major sources of error in a previously used method and consists of first igniting the sample to remove all organic matter and most of the carbon dioxide and to oxidize any iron compounds to the ferric state. This is followed by solution of the sample under mild conditions and volumetrically determining the calcium and magnesium under conditions which eliminate the effect of any dissolved impurities.