The "feedback" from studies of Recent carbonate sediments to studies of ancient carbonate rocks is small. The reason for this unfortunate state of affairs is that the approaches used in the two problems are so different. For example, the materials are different; carbonate sediments are conventionally nearly pure carbonate, whereas carbonate rocks contain up to 50 per cent non-carbonates.

Sediment studies usually involve rather elaborate size analyses, mineralogic studies, and individual particle descriptions. For most carbonate rocks diagenetic alteration has made size analysis difficult or meaningless, the minerals have changed, and only a part of the original particles can even be recognized.

The best understanding of carbonate rocks comes from conventional isopachous maps, stratigraphic associations, insoluble-residue studies, and fossil content. Only in a few cases can petrographic studies of size or particle type do more than support conclusions based on field relations.

The writers do not see any immediate way of changing drastically the methods for studying carbonate rocks. However, new approaches are possible in the study of sediments. By simply enlarging the

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definition of carbonate sediments to include those containing more than 50 per cent carbonate minerals, many new parameters can be studied and new and useful information can be incorporated in studies of carbonate rocks.

It is obvious that, by enlarging the definition, consideration of insoluble residues and stratigraphic associations is possible. Further, the geographic range of carbonate sediments is enlarged, permitting study of a wider variety of environmental conditions. Finally, a greater variation in thickness of Recent carbonate sediments is found, a fact which should allow a better understanding of sedimentation rates.

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